U.S. patent application number 13/560018 was filed with the patent office on 2013-08-01 for process and apparatus for adsorptive bubble separation.
This patent application is currently assigned to RENEWABLE ALGAL ENERGY, LLC. The applicant listed for this patent is C. Calvert Churn, III, Robert L. Clayton, Stephen N. Falling, Jeffrey S. Kanel. Invention is credited to C. Calvert Churn, III, Robert L. Clayton, Stephen N. Falling, Jeffrey S. Kanel.
Application Number | 20130193036 13/560018 |
Document ID | / |
Family ID | 40156557 |
Filed Date | 2013-08-01 |
United States Patent
Application |
20130193036 |
Kind Code |
A1 |
Clayton; Robert L. ; et
al. |
August 1, 2013 |
PROCESS AND APPARATUS FOR ADSORPTIVE BUBBLE SEPARATION
Abstract
Process and apparatus are described for adsorptive bubble
separation of hydrophobic particles from liquid dispersions. When a
gas-liquid-particle dispersion is introduced into a separation
vessel, a baffle directs the rising bubbles toward the perimeter of
the apparatus. At the liquid surface, bubbles with attached
hydrophobic materials form a floating froth layer, which is
directed toward a froth collection launder. Also disclosed is an
improvement for froth flotation processes comprising using a vacuum
to pull froth and/or collapsed froth into and through the froth
collection launder and froth drain line.
Inventors: |
Clayton; Robert L.; (Tuscon,
AZ) ; Falling; Stephen N.; (Kingsport, TN) ;
Kanel; Jeffrey S.; (Kingsport, TN) ; Churn, III; C.
Calvert; (Kingsport, TN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Clayton; Robert L.
Falling; Stephen N.
Kanel; Jeffrey S.
Churn, III; C. Calvert |
Tuscon
Kingsport
Kingsport
Kingsport |
AZ
TN
TN
TN |
US
US
US
US |
|
|
Assignee: |
RENEWABLE ALGAL ENERGY, LLC
Kingsport
TN
EASTMAN CHEMICAL COMPANY
Kingsport
TN
|
Family ID: |
40156557 |
Appl. No.: |
13/560018 |
Filed: |
July 27, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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12665211 |
Dec 17, 2009 |
8251228 |
|
|
PCT/US2008/007613 |
Jun 18, 2008 |
|
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13560018 |
|
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60944813 |
Jun 19, 2007 |
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Current U.S.
Class: |
209/164 ;
209/168 |
Current CPC
Class: |
C02F 1/24 20130101; B03D
1/1412 20130101; C02F 2303/12 20130101; B03D 1/20 20130101; C02F
2201/003 20130101; C02F 2301/063 20130101; C02F 1/40 20130101; B03D
1/24 20130101; C02F 2209/40 20130101; B03D 1/02 20130101; C02F 1/52
20130101; C02F 2209/38 20130101; C12N 1/02 20130101; B03D 1/1462
20130101; C02F 2101/32 20130101; C02F 2103/10 20130101; C12M 47/02
20130101; C12N 1/066 20130101; C02F 2101/20 20130101 |
Class at
Publication: |
209/164 ;
209/168 |
International
Class: |
B03D 1/14 20060101
B03D001/14; B03D 1/24 20060101 B03D001/24; B03D 1/02 20060101
B03D001/02 |
Claims
1-14. (canceled)
15. An adsorptive bubble separation process of the type involving
froth flotation wherein the floating froth flows into a collection
launder, wherein the improvement comprises: directing rising
bubbles and falling dislodged particles to the perimeter of a
flotation chamber with one or more baffles; and forcing the
floating froth to flow on the liquid surface to a region of reduced
surface area before overflowing into the collection launder.
16. An adsorptive bubble separation process of the type involving
froth flotation wherein the floating froth flows into a collection
launder, wherein the improvement comprises: directing rising
bubbles and falling dislodged particles to the perimeter of a
flotation chamber with one or more baffles; and utilizing a vacuum
to pull and/or collapse froth through the collection launder.
17. A method of concentrating particles in a liquid-particle
dispersion by adsorptive bubble separation, the method comprising:
introducing the liquid-particle dispersion into a vessel at a point
below a surface of a liquid contained therein; introducing a gas
into the vessel at a point below the introduction point of the
liquid-particle dispersion; forming bubbles comprising a
gas-particle agglomerate; directing the bubbles to the surface of
the liquid with one or more baffles so as to form a floating froth,
wherein the froth is enriched in particles and wherein the one or
more baffles direct the rising bubbles and falling dislodged
particles to the perimeter of the vessel; directing the froth
towards a froth collection launder in a region of lower surface
area than the region where it formed; and collecting the froth in
the froth collection launder.
18. The method according to claim 17, wherein the froth collection
launder is located substantially in the center of the vessel.
19-20. (canceled)
21. A froth flotation device of the type wherein froth floating in
a vessel flows into a collection launder for collection, the
improvement comprising: incorporating one or more baffles in the
vessel to direct rising bubbles and falling dislodged particles to
the perimeter of the vessel; and incorporating into the froth
flotation device a vacuum generating device to pull and/or collapse
froth through the collection launder.
22. The froth flotation device of claim 21, wherein vacuum is
generated by a pump, compressor, aspirator, venturi, eductor,
and/or blower used to provide gas introduced into the vessel for
preparation of bubbles for froth.
23. A froth flotation device of the type wherein froth floating in
a vessel flows into a collection launder for collection, the
improvement comprising: one or more baffles in the vessel for
directing rising bubbles and falling dislodged particles to the
perimeter of the vessel; and a collection launder centrally located
within the vessel.
Description
REFERENCE TO RELATED APPLICATION
[0001] This application claims priority under 35 U.S.C.
.sctn.119(e) to U.S. Provisional Patent application 60/944,813,
filed on Jun. 19, 2007, the contents of the entirety of which are
incorporated herein by this reference.
BACKGROUND
[0002] Adsorptive bubble separation (which includes froth
flotation, flotation, bubble fractionation, dissolved air
flotation, and solvent sublation) is a process in which a
molecular, colloidal or particulate material is selectively
adsorbed to the surface of gas bubbles rising through a liquid, and
is thereby concentrated or separated. A commonly used type of
adsorptive bubble separation process is froth flotation wherein the
bubble-particle agglomerates accumulate on the liquid surface as a
floating froth. The froth with adsorbed (i.e., attached or
collected) particles is treated in one of several ways to collapse
the froth and isolate the material. See for example, Flotation
Science and Engineering, K. A. Mattis, Editor, pages 1 to 44,
Marcel Dekker, New York, N.Y., 1995; and Adsorptive Bubble
Separation Techniques, Robert Lemlich, Editor, pages 1 to 5,
Academic Press, New York, N.Y., 1972.
[0003] This important process is commercially utilized in a wide
range of applications including: isolation of minerals and metals
from an ore-water slurry, dewatering of microalgae, yeast or
bacterial cells, removal of oil from water, removal of ash from
coal, removal of particles in waste-water treatment streams,
purification of drinking water, and removal of ink and adhesives
during paper recycling. In most applications, it is necessary to
add reagents, known as "collectors", which selectively render one
or more of the species of particles in the feed hydrophobic,
thereby assisting in the process of collection by the gas bubbles.
It is also not unusual to add frothing agents to assist in the
formation of a stable froth on the surface of the liquid. The
process of admitting these various reagents to the system is known
as conditioning. The feed for the adsorptive bubble separation
process may be a mixture, dispersion, emulsion, slurry, or
suspension of a molecular, colloidal and/or particulate material in
a liquid and is referred to hereafter as the liquid-particle
dispersion. When the liquid is water, as is usually the case, the
feed may be referred to as an aqueous-particle dispersion.
[0004] Because of the importance of adsorptive bubble separation
processes, there have been many attempts to improve the efficiency
and selectivity of particle capture from an aqueous-particle
dispersion in order to increase product yield and purity.
[0005] U.S. Pat. Nos. 4,668,382, 4,938,865, 5,332,100, and
5,188,726 (the contents of the entirety of each of which are
incorporated herein by this reference) disclose an adsorptive
bubble separation process and apparatus (commonly known as a
"Jameson cell") wherein an aqueous-particle dispersion enters the
top of a vertical duct (downcomer) and passes through an orifice
plate to form a high velocity, downward facing liquid jet. A gas,
usually air introduced into the downcomer headspace, is dispersed
into the mixture as the liquid jet impacts a foam column within the
downcomer. The volume within the downcomer is referred to as the
collection zone wherein most of the particles adsorb to the surface
of the bubbles. The resulting gas-liquid-particle dispersion exits
through the bottom of the downcomer into the separation zone where
the bubbles separate from the tails (water and non-adsorbed
materials). In the separation zone, the gas-liquid-particle
dispersion has sufficient residence time to allow the tiny bubbles
with collected particles to coalesce (combine and enlarge) and rise
to the liquid surface forming a particle-rich, floating froth in
the froth zone. The froth is collected by allowing it to float
outward to the perimeter of the apparatus and overflow into an open
launder (collection trough). Provisions are made in these patents
to incorporate froth washing in the froth zone by introducing a
liquid onto the froth from above thus creating a net downward
liquid flow and washing the entrained gangue (undesired solid
matter) and non-adsorbed particles away from the froth. This
washing produces a purer froth, and therefore a more selective
separation. In the design described in these patents, the washing
occurs over the whole surface of the froth rather than in a focused
region of the froth.
[0006] In addition, U.S. Pat. No. 4,668,382 (the contents of the
entirety of which are incorporated herein by this reference)
changes the configuration from a tank with vertical walls to
converging walls so that the froth is squeezed (crowded) as it
collects on the liquid surface. This allows for a higher froth
depth than would normally occur, thus permitting better collection
selectivity in the portion of froth overflowing into the collection
launder. This design however requires an expensive fabrication
process to make the converging sides.
[0007] U.S. Pat. No. 6,832,690 (the contents of the entirety of
which are incorporated herein by this reference) also describes a
method of squeezing the froth in a. complex geometry, while U.S.
Pat. No. 5,251,764 (the contents of the entirety of which are
incorporated herein by this reference) describes a complex
hydraulically-operated system. Froth zone surface fouling can be
troublesome in these modifications of the original Jameson cell
design.
[0008] In column flotation cells such as the MICROCEL.TM., U.S.
Pat. Nos. 4,981,582 and 5,167,798; the Deister Column Cell, U.S.
Pat. No. 5,078,921; and the Multistage Loop-Flow Flotation
(MSTLFLO) column, U.S. Pat. No. 5,897,772 (the contents of the
entirety of each of which are incorporated herein by this
reference), the collection, separation, and froth zones and froth
washing are combined in a tall, cylindrical tank, which is less
effective and more expensive to construct. In these column
flotation cells, the froth at the top of the column overflows into
an outer launder that surrounds the column. Sometimes an additional
central launder is added to increase the froth discharge area when
it is necessary to achieve rapid removal of voluminous froth.
[0009] Mechanical flotation cells typically employ a rotor and
stator mechanism for gas induction, bubble generation, and liquid
circulation thus providing for bubble and particle collision. The
ratio of vessel height to diameter, termed the "aspect ratio",
usually varies from about 0.7 to 2. Typically, four or more cells
each having a centrally mounted rotor and stator mechanism are
arranged in series. The liquid-particle dispersion is fed into the
cell and air is sucked into the cell through a hollow shaft
agitator. The air stream is broken by the rotating impeller, so
that small bubbles are emitted from the end of the impeller blades.
An auxiliary blower may also be used to provide sufficient gas flow
to the cell. Rising bubbles together with attached particles form a
froth layer on the top of the liquid surface. The froth layer
overflows or is skimmed off mechanically from the top. Non-floated
components are withdrawn from the bottom of the cell. Mechanical
flotation cells are often used in mineral processing systems;
however they have the disadvantage of large space requirements,
long liquid residence times, and high power consumption.
[0010] For example, U.S. Pat. Nos. 4,425,232 and 4,800,017 (the
contents of the entirety of which are incorporated herein by this
reference) describe mechanical flotation separation utilizing a
flotation cell provided with a rotor-stator assembly submerged in a
slurry and in which rotor blades agitate the slurry thoroughly
mixing the solids and liquid and introducing air to the mixture for
aeration and generation of froth on the liquid surface. Particles
of minerals attach to carrier air bubbles which are naturally
buoyant and form the froth, this being the effective mechanism for
mineral recovery. The floating froth is removed from the top of the
slurry together with the attached mineral particles which are
recovered as froth is collapsed and dewatered.
[0011] In all of these previously described processes, the desired
particles that have prematurely disengaged (i.e., desorbed or
detached) from the bubbles are inefficiently contacted with rising
gas bubbles over the entire cross sectional area of the tank, thus
lowering the chance of recapturing them. In addition, these designs
typically have froth collection launders around the perimeter,
which reduces the froth density as the froth spreads from the
center outward (from low surface area to high surface area) thereby
reducing the froth height and the selectivity of froth
overflow.
SUMMARY OF THE INVENTION
[0012] Described is a highly efficient process and apparatus
(flotation cell) for increasing the collection effectiveness of
bubbles and improving the purity of the froth produced in an
adsorptive bubble separation process, in which collected
hydrophobic materials (particles) are attached to the bubbles.
These materials typically include solids, liquids, or both. Above
the froth-liquid interface within the flotation cell is the froth
zone wherein the bubble-particle agglomerates form a floating froth
layer. By the design disclosed herein, this froth naturally floats
toward an open central froth collection launder into which it
overflows. The action of the rising bubbles at the perimeter
pushing the froth layer toward the reduced surface area of the
center squeezes (crowds) the froth causing bubble coalescence and
increased liquid drainage thereby achieving an increased
concentration of collected materials.
[0013] The improved adsorptive bubble separation design may be
utilized in any flotation cell by forcing floating froth to flow on
the liquid surface to a region of lower surface area before
overflowing into a collection launder. This improved froth
collection design may be utilized in the operation of mechanical
flotation cells, pneumatic flotation cells such as the Jameson
cell, Multistage Loop-Flow flotation columns, and bubble flotation
columns (also known as "Canadian Columns") by replacement of the
their perimeter collection launder with a central collection
launder.
[0014] As a consequence of the design, the length of the collection
launder lip (referring to the launder edge which the froth
overflows) is shorter than the length of the perimeter of the
separation apparatus. This is in contrast to processes of the prior
art wherein the collection launder is located around the perimeter
of the apparatus so that the launder lip length is the same length
as the perimeter. In those prior art designs where a central
launder is also used, the launder lip length is further increased.
The design of the invention is especially useful in the recovery or
removal of low concentrations of hydrophobic materials in water. In
oil recovery from water, for example, it is desirable to
concentrate the oil in the froth to the greatest extent possible
before it is removed from the flotation cell. This design is also
useful for the dewatering of microalgae in very dilute microalgal
cultures.
[0015] In any adsorptive bubble separation process, a portion of
the desired hydrophobic material is not captured by, or is
dislodged from, the rising bubbles. An optional performance
enhancement in the instant design provides a means for forcing
re-contact of these disengaged particles with rising bubbles at the
perimeter. This enhancement is achieved by the use of a baffle in
the separation vessel which causes the disengaged particles to flow
down and outward with the liquid draining from the froth to
re-contact the rising bubbles at the perimeter. This re-contact
with bubbles encourages re-adsorption of the desired hydrophobic
material resulting in better recovery than generally obtained in
the prior art.
[0016] The process for adsorptive bubble separation can be repeated
one or more times in order to affect an efficient countercurrent
flow of froth and draining liquid for highly efficient particle
capture. The process for adsorptive bubble separation can be
operated batchwise or continuously. Continuous operation is
preferred in most applications.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is schematic block diagram of the process.
[0018] FIG. 2 is a sectional view of a cylindrical separation
apparatus in which the gas-liquid-particle dispersion introduction
ducts are arranged near the perimeter of the vessel and the froth
collection launder is in the center.
[0019] FIG. 3 is a top view of the apparatus of FIG. 2.
[0020] FIG. 4 is a sectional view of a cylindrical separation
apparatus in which the gas-liquid-particle dispersion introduction
ducts are arranged near the perimeter of the vessel, the froth
collection launder is in the center and a baffle is used to direct
disengaged particles back to the perimeter for re-contact with
rising bubbles.
[0021] FIG. 5 is a sectional view of a cylindrical separation
apparatus in which a single gas-liquid-particle dispersion
introduction duct is located in the center and the central froth
collection launder is attached to the duct in an annular fashion, a
baffle is used to direct the rising bubbles to the perimeter.
[0022] FIG. 6 is an illustration of how a conventional bubble
column can be modified with a central froth launder to utilize the
design of the invention.
[0023] FIG. 7 is an illustration of how a mechanical flotation cell
can be modified with a central froth launder to utilize the design
of the invention.
[0024] FIG. 8 is a sectional view illustrating a rectangular
separation apparatus with gas-liquid-particle dispersion ducts
arranged on the diagonal and froth collection launders in the
opposite corners.
[0025] FIG. 9 is a top view of the rectangular apparatus of FIG.
8.
[0026] FIG. 10 is a sectional view illustrating an optional
vacuum-aided froth collection system and an optional spray-aided
froth collection system.
[0027] FIG. 11 is a top view of a cylindrical flotation cell of the
type shown in FIGS. 2 and 3 which illustrates how the floating
froth crowds into areas of lower surface area before spilling into
the central froth collection launder.
DETAILED DESCRIPTION OF THE INVENTION
[0028] As shown in FIG. 1, the froth flotation device includes a
bubble generation zone 3, a collection zone 4, a separation zone 5,
and a froth zone 7. Some or all of these zones may or may not
occupy the same vessel. The liquid-particle dispersion feed 1
enters the froth flotation device at either the collection zone 4
or the bubble generation zone 3 or both, depending on the equipment
chosen. In either event, a gas 2 is dispersed through the bubble
generation zone 3 and/or the collection zone 4 to produce a
gas-liquid-particle dispersion. It is desirable to produce a large
number of small bubbles to maximize the surface area of gas
available for collision with hydrophobic particles in a given
volume of the feed dispersion 1.
[0029] In the collection zone 4, the hydrophobic particles are
mixed with the fine bubbles under conditions that promote intimate
contact to produce the gas-liquid-particle dispersion. The bubbles
collide with the hydrophobic particles and form bubble-particle
agglomerates. It is desirable to generate intense mixing in the
collection zone 4 to cause a high frequency of collisions in order
to achieve high particle capture efficiency.
[0030] After the bubble-particle agglomerates are formed in the
collection zone 4, they are then separated from the
particle-depleted liquid in the separation zone 5, typically by
gravity. The density of the gas is generally at least two to three
orders of magnitude less than that of the liquid. The density
difference promotes floating of the bubble-particle agglomerates to
the surface of the liquid, where the agglomerates accumulate as a
froth in the froth zone 7.
[0031] The froth, enriched in hydrophobic particles, overflows the
froth zone 7 as stream 8. The underflow stream (tails) 6, which is
the liquid depleted in hydrophobic particles, exits the froth
flotation device and may be treated again in a secondary flotation
cell, recycled or discarded.
[0032] FIG. 2 shows an apparatus for adsorptive bubble separation
in which the gas-liquid-particle dispersion 10 is introduced below
the liquid surface (froth-liquid interface) 11 through introduction
ducts 12 into the separation zone 13. In all embodiments of this
invention the gas may comprise air, nitrogen, argon, helium, carbon
dioxide, gas from the combustion of carbonaceous material, solvent
vapor, carbon dioxide from a gasification plant, or combinations
thereof. The gas may also be pre-saturated with liquids, especially
liquids that are contained in the feed dispersion. The
gas-liquid-particle dispersion 10 can be produced by methods known
in the prior art such as aspiration of the gas into the
liquid-particle dispersion using an eductor, a plunging jet, or an
agitated system.
[0033] The introduction ducts 12 introducing the
gas-liquid-particle dispersion 10 may be vertical, have a vertical
section, or be essentially vertical so that it contains the column
of foam in a manner so that sufficient vacuum is maintained in the
top of the duct to maintain a column of at least some of the
gas-liquid-particle dispersion in the duct. Alternatively (or
additionally), the introduction ducts may enter the flotation cell
through the side of the vessel. This embodiment may also have two
or more introduction ducts 12 evenly spaced near the perimeter of
the vessel. It is preferred to have four or more introduction ducts
12 evenly spaced near the perimeter of the vessel.
[0034] The gas-liquid-particle dispersion 10 exiting from the
introduction ducts 12 has been created such that the
liquid-particle dispersion feed has been brought into intimate
contact with the gas with enough energy and for sufficient time
that an acceptable percentage of the hydrophobic material has been
collected on the bubbles. The introduction ducts 12 may be
cylindrical in shape, but other geometries could be used, including
but not limited to rectangles, squares, ovals, triangles, and other
polygons. The introduction ducts 12 may be constructed from any
material used in the art, including but not limited to polyvinyl
chloride (PVC), high-density polyethylene (HDPE), polycarbonate,
other polymers, glass, fiberglass, steel, iron, other metals,
concrete, tile, or other construction materials.
[0035] The bottoms of the introduction ducts 12 are submerged below
the surface of the liquid 11 within the separation zone 13 where
the exiting gas-liquid-particle dispersion 10 begins to coalesce
into larger bubbles 14 and rise toward the liquid surface 11
carrying the collected materials. The separation zone 13 can be
configured in any shape so long as the residence time is
sufficiently great to allow for bubble coalescence and the
separation of the bubble-particle agglomerates and liquid stream.
Even though the separation zone 13 may be cylindrical, square,
rectangular, hexagonal, or other shape, it is preferable to use a
cylindrical design. The outer wall 15 of the flotation cell may be
constructed from any material used in the art, including but not
limited to polyvinyl chloride (PVC), high-density polyethylene
(HDPE), polycarbonate, other polymers, glass, fiberglass, steel,
iron, other metals, concrete, tile, earth, stone, or other
construction materials.
[0036] The rising bubble-particle agglomerates 14 accumulate as a
froth 17 above the liquid surface (froth-liquid interface) 11 in
the froth zone 16. In this zone, the froth continues to drain,
purifying the froth and concentrating the collected material. As
additional bubbles rise forming more froth, they push the
accumulated froth on the surface toward the center where the upper
portion overflows the launder lip into the froth collection launder
9. This movement toward the reduced surface area of the center
squeezes the froth helping to cleanse and drain it. Wash water may
be added to the froth from above if desired to further purify it.
Any suitable liquid can be used for the froth washing operation.
Suitable liquids include, but are not limited to water, liquids
that are native to the feed dispersion, solutions of surface
treatment and conditioning agents, and combinations thereof.
[0037] The purified froth 17 overflows into a central collection
launder 9 that extends above the liquid surface level 11. The
collection launder 9 can be of any shape, but it is preferably the
same geometric shape as the flotation cell. The central collection
launder 9 is most preferably a circular pipe or hollow column. The
collection launder 9 may be constructed from any material used in
the art, including but not limited to polyvinyl chloride (PVC),
high-density polyethylene (HDPE), polycarbonate, other polymers,
glass, fiberglass, steel, iron, other metals, concrete, tile, or
other construction materials. The froth and collapsed froth 18
drains down the collection launder 9 and then exits the flotation
cell through the bottom or the side via a drain line 19.
[0038] The feed liquid depleted in hydrophobic particles (tails) 20
underflows the flotation device through a bottom or side tails line
21 and may be treated again in a secondary flotation cell, recycled
or discarded. The bottom of the flotation cell may be flat,
hemispherical or conical. In processes in which solids settle to
the bottom, a sloped-flat, hemispherical or conical bottom with
bottom tails line 21 is desired for improved solids removal. The
liquid level 11 within the flotation cell may be controlled by
controlling the liquid flow through the tails line 21. Optionally,
liquid level control may be conveniently maintained without valves
or control devices by the use of one or more overflow side arms or
swing arms.
[0039] FIG. 3 shows a top view of the apparatus for adsorptive
bubble separation of FIG. 2 employing four introduction ducts
12.
[0040] FIG. 4 shows an optional performance enhancement to the
apparatus for adsorptive bubble separation of FIGS. 2 and 3. This
optional enhancement includes a baffle 23 to direct the disengaged
particles to flow down and outward with the liquid draining from
the froth to re-contact the rising bubbles at the perimeter. This
re-contact with bubbles encourages re-adsorption of the desired
hydrophobic material resulting in better recovery than would be
obtained by methods in the prior art.
[0041] A froth baffle 23 directs the bubbles 24 outward to the gap
25 between the baffle 23 and the perimeter wall 26 where they then
rise toward the liquid surface 27. Likewise disengaged particles
that are sinking are directed outward by baffle 23 to the gap 25
where they will be re-contacted with rising bubbles. The froth
baffle 23 may be of any shape which directs the rising bubbles and
the sinking particles to a location near the perimeter. The baffle
23 may be conical, flat, tapered, or sloped and constructed from
any material used in the art, including but not limited to PVC,
HDPE, polycarbonate, rubber, other polymers, glass, fiberglass,
steel, iron, other metals, concrete, tile, or other construction
materials.
[0042] FIG. 5 shows an apparatus for adsorptive bubble separation
in which the gas-liquid-particle dispersion 28 is introduced below
the liquid surface 29 through one central introduction duct 30 into
the separation zone 31. A froth baffle 32 directs the bubbles 33
outward to the gap 34 between the baffle 32 and the perimeter wall
35 where they then rise toward the liquid surface 29. Likewise
disengaged particles 36 that are sinking are directed outward by
baffle 32 to the gap 34 where they will be re-contacted with rising
bubbles. The froth baffle 32 may be of any shape which directs the
rising bubbles and the sinking particles to a location near the
perimeter. The baffle may be conical, flat, tapered, or sloped and
constructed from any material used in the art, including but not
limited to PVC, HDPE, polycarbonate, rubber, other polymers, glass,
fiberglass, steel, iron, other metals, concrete, tile, or other
construction materials.
[0043] The rising bubble-particle agglomerates accumulate, as a
froth 37, above the liquid surface 29 in the froth zone 38. In this
zone, the froth continues to drain, purifying the froth and
concentrating the collected material. As additional bubbles rise
forming more froth, they push the accumulated froth on the surface
toward the center where the upper portion overflows the launder lip
into the collection launder 39. This movement toward the reduced
surface area of the center squeezes the froth helping to cleanse
and drain it. Wash water may be added to the froth from above if
desired to further purify it.
[0044] The purified froth 37 overflows into a central collection
launder 39 which extends above the liquid surface level 29. The
collection launder 39 can be of any shape, but it is preferred that
the collection launder 39 be an annulus around a tubular
introduction duct 30. The collection launder 39 may be constructed
from any material used in the art, including but not limited to
polyvinyl chloride (PVC), high-density polyethylene (HDPE),
polycarbonate, other polymers, glass, fiberglass, steel, iron,
other metals, concrete, tile, or other construction materials. The
froth or collapsed froth 37 drains down the collection launder 39
and then exits the flotation cell through the bottom or the side
via a drain line 40.
[0045] The feed liquid depleted in hydrophobic particles (tails) 41
underflows the flotation device through a bottom or side tails line
42 and may be treated again in a secondary flotation cell, recycled
or discarded. The bottom of the flotation cell may be flat,
hemispherical or conical. In processes in which solids settle to
the bottom, a sloped-flat, hemispherical or conical bottom with
bottom tails line 42 is desired for improved solids removal. The
liquid level 29 within the flotation cell may be controlled by
controlling the liquid flow through the tails line 42. Optionally,
the liquid level 29 may be maintained by the use of one or more
self-leveling devices such as an overflow side arm or swing
arm.
[0046] One of the design parameters in flotation cell design is Jg
(the superficial gas rise rate), and it is typically calculated by
dividing the gas flow rate entering the cell by the cell area. High
Jg rates (greater than 1 cm per second) will typically produce high
recovery as a rapid bubble rise rate leaves less time for the
bubbles to coalesce and particles to disengage. In this rising
stream, gangue and liquid can be entrained in the flow also. Lower
Jg rates (less than 1 centimeter per second) allow more time for
bubble coalescence and drainage of the froth to produce a more pure
froth. An even rise of bubbles is assumed in the calculation of Jg.
In the prior art, it is understood that uniformity in the froth
flow path results in uniform treatment of the froth and gives more
predictable performance. With a central froth baffle in a
cylindrical tank, froth distribution takes place in a radial
direction with constant distances and a uniform inward froth flow
path throughout 360 degrees.
[0047] FIG. 6 shows a flotation column embodiment of the invention
in which the collection, separation, and froth zones and optional
froth washing are combined in a tall, cylindrical tank 43. This
design is an improvement of the conventional bubble column which is
also known as the "Canadian Column". The cross section may be
circular, square or hexagonal. The liquid-particle dispersion
enters the column at a point below the liquid surface (froth-liquid
interface) 44 through line 45. The gas enters the base of the
column through line 46 and is dispersed into fine bubbles,
typically by means of a sparger 47 or it is introduced as an
aerated liquid.
[0048] The countercurrent flow of gas and feed dispersion results
in bubble and particle collision in the collection zone 48 which is
defined as the region below the feed distributor 49. The separation
zone 50 for the column is above the feed distributor 49 and below
the froth-liquid interface 44.
[0049] The rising bubble-particle agglomerates accumulate as a
froth 51 above the liquid surface 44 in the froth zone 52. In this
zone the froth continues to drain, purifying the froth and
concentrating the collected material. As additional bubbles rise
forming more froth, they push the accumulated froth on the surface
toward the reduced surface area of the center where the upper
portion overflows the launder lip into the collection launder 53.
This movement toward the center squeezes the froth helping to
cleanse and drain it. Wash water may be added to the froth from
above if desired to further purify it.
[0050] The purified froth 51 overflows into a central collection
launder 53 which extends above the froth-liquid interface 44. The
collection launder 53 can be of any shape, but it is preferably the
same geometric shape as the flotation cell. The central collection
launder 53 is most preferably a circular pipe or hollow column. The
collection launder 53 may be constructed from any material used in
the art, including but not limited to polyvinyl chloride (PVC),
high-density polyethylene (HDPE), polycarbonate, other polymers,
glass, fiberglass, steel, iron, other metals, concrete, tile, or
other construction materials.
[0051] The froth or collapsed froth 51 drains down the collection
launder 53 and then exits the flotation cell through the bottom or
the side via a drain line 54. The tails, depleted of hydrophobic
particles, underflows the column through a bottom or side tails
line 55 and may be treated again in a secondary flotation cell,
recycled or discarded. The bottom of the flotation cell may be
flat, hemispherical or conical. In processes in which solids settle
to the bottom, a sloped-flat, hemispherical or conical bottom with
bottom tails line 55 is desired for improved solids removal. The
liquid level 44 within the flotation cell may be controlled by
controlling the liquid flow through the tails line 55. Optionally,
the liquid level 44 may be maintained by the use of one or more
self-leveling devices such as an overflow side arm or swing
arm.
[0052] FIG. 7, shows a mechanical flotation cell embodiment of the
invention in which the bubble generation zone 57, collection zone
58, separation zone 59, and froth zone 60 are combined into a
single large tank 56. Mechanical cells typically employ a rotor and
stator mechanism 61 for gas induction, bubble generation, and
liquid circulation providing for bubble and particle collision.
Typically, four or more cells similar to that in FIG. 7, each
having a centrally mounted rotor and stator mechanism 61, are
arranged in series to improve efficiency. An auxiliary blower is
occasionally installed to provide sufficient gas flow to the
cell.
[0053] The gas is dispersed into fine bubbles by a rotating
impeller 62, which serves as the bubble generator. The rotating
impeller creates a low pressure zone that induces gas to flow
through an aspiration tube 63 into the collection zone 58 where it
is dispersed into fine bubbles and mixed with the liquid-particle
dispersion as it circulates in the bottom of the cell.
[0054] The properly designed rotor and stator mechanism entrains
the proper amount of gas, disperses it into fine bubbles, and mixes
the gas with liquid to accomplish sufficient contact between the
particles and the bubbles. Good mixing and sufficient liquid
residence time are necessary in the two phase mixing region to
provide high bubble and particle collision efficiency, and good
flotation performance. Rotor and stator mechanisms include those
produced by Dorr-Oliver Incorporated of Millford, Conn.; Denver
Equipment Company which is a division of Svedala of Colorado
Springs, Colo.; Wemco Products of Salt Lake City, Utah; and Outomec
Oy of Espoo, Finland.
[0055] The liquid dispersion enters the mechanical cell as a feed
stream 64 through a feed box 65. Bubble and particle contact
results from turbulence generated by the rotating impeller 62. The
bubbles with attached particles pass out of the collection zone 58
into the separation zone 59, which is relatively quiescent, where
they float to the surface and separate from the liquid phase.
[0056] The rising bubbles accumulate as a froth 66 above the liquid
surface (froth-liquid interface) 67 in the froth zone 60. In this
zone the froth continues to drain, purifying the froth and
concentrating the collected material. As additional bubbles rise
forming more froth, they push the accumulated froth on the surface
toward the reduced surface area of the center where the upper
portion overflows the launder lip into the collection launder 68.
This movement toward the center squeezes the froth helping to
cleanse and drain it. Wash water may be added to the froth from
above if desired to further purify it.
[0057] The purified froth 66 overflows into a central collection
launder 68 that extends above the liquid surface level 67. The
collection launder 68 can be of any shape, but it is preferred that
the collection launder 68 be an annulus around a circular
aspiration tube 63 and rotor shaft 69. The collection launder 68
may be constructed from any material used in the art, including but
not limited to PVC, HDPE, polycarbonate, other polymers, glass,
fiberglass, steel, iron, other metals, concrete, tile, or other
construction materials. The froth or collapsed froth drains down
the collection launder 68 and then exits the flotation cell through
the bottom or the side via a drain line 70.
[0058] The liquid phase recirculates in the collection zone 58, but
eventually exits the cell as an underflow stream 71 through a
bottom or side tails line 72. The tails may be treated again in a
secondary flotation cell, recycled or discarded. The bottom of the
flotation cell may be flat, hemispherical or conical. In processes
in which solids settle to the bottom, a sloped-flat, hemispherical
or conical bottom with bottom tails line 72 is desired for improved
solids removal. The liquid level 67 within the flotation cell may
be controlled by controlling the liquid flow through the tails line
72. Optionally, the liquid level 67 may be maintained by the use of
one or more self-leveling devices such as an overflow side arm or
swing arm.
[0059] FIG. 8, shows an example of how our discovery for improving
froth purity by causing the froth to squeeze into a region of lower
surface area can be utilized in alternate flotation cell
geometries. FIG. 8 shows a rectangular, or preferably square,
flotation cell embodiment of the invention in which the
gas-liquid-particle dispersion 73 is introduced below the liquid
surface (froth-liquid interface) 74 through two or more
introduction ducts 75 into the separation zone 76. It is preferred
in this embodiment to have two or more introduction ducts 75 evenly
spaced along a diagonal of the vessel. The introduction ducts 75
may be constructed from any material used in the art, including but
not limited to polyvinyl chloride (PVC), high-density polyethylene
(HDPE), polycarbonate, other polymers, glass, fiberglass, steel,
iron, other metals, concrete, tile, or other construction
materials.
[0060] The bottoms of the introduction ducts 75 are submerged below
the surface of the liquid 74 within the separation zone 76 where
the exiting gas-liquid-particle dispersion 73 begins to coalesce
into larger bubbles and rise carrying the collected materials. The
separation zone 76 in this embodiment is square or rectangular and
of such height as to allow for bubble coalescence and the
separation of froth and liquid streams. The outer wall 77 of the
flotation cell may be constructed from any material used in the
art, including but not limited to PVC, HDPE, polycarbonate, other
polymers, glass, fiberglass, steel, iron, other metals, concrete,
tile, earth, stone, or other construction materials.
[0061] The rising bubble-particle agglomerates accumulate as a
froth 78 above the liquid surface (froth-liquid interface) 74 in
the froth zone 79. In this zone the froth continues to drain,
purifying the froth and concentrating the collected material. As
additional bubbles rise forming more froth, they push the
accumulated froth on the surface toward the two corners where the
upper portion overflows the launder lips into the collection
launders 80. This movement toward the lower-surface-area corners
squeezes the froth helping to cleanse and drain it. Wash water may
be added to the froth from above if desired to further purify
it.
[0062] The purified froth 78 overflows into the corner collection
launders 80 which extend above the liquid surface level 74. The
collection launders 80 may be any shape although shapes that fit
tightly in the corner are preferred. These shapes would include
square, rectangular, triangular, quarter-circle and circular. The
collection launders 80 may be constructed from any material used in
the art, including but not limited to PVC, HDPE, polycarbonate,
other polymers, glass, fiberglass, steel, iron, other metals,
concrete, tile, or other construction materials. The froth or
collapsed froth 78 drains down the collection launders 80 and then
exits the flotation cell through the bottom or the side via drain
lines 81. Alternatively, the collection launders in this
rectangular embodiment may also be exterior to the flotation cell
by the use of a corner notch which serves as the froth collection
launder lip.
[0063] The feed liquid depleted in hydrophobic particles (tails) 82
underflows the flotation device through a bottom or side tails line
83 and may be treated again in a secondary flotation cell, recycled
or discarded. The bottom of the flotation cell may be flat,
hemispherical or conical. In processes in which solids settle to
the bottom, a sloped-flat, hemispherical or conical bottom with
bottom tails line 83 is desired for improved solids removal. The
liquid level 74 within the flotation cell may be controlled by
controlling the liquid flow through the tails line 83. Optionally,
liquid level control may be conveniently maintained without valves
or control devices by the use of one or more overflow side arms or
swing arms.
[0064] FIG. 9 shows a top view of the flotation cell embodiment of
FIG. 8 employing three introduction ducts 75.
[0065] In the process of the invention the feed dispersion
comprises particles and a carrier fluid, usually water. The
particles may comprise solid particles or liquid droplets, or
combinations thereof. Examples of the solid particles include, but
are not limited to minerals, gangue, micro-organisms, coal, inks,
pigments, or combinations thereof. Examples of liquid droplets
include, but are not limited to organic solvents, metal extraction
solvents, dyes, inks, oils, hydrocarbons, fuels, triglycerides,
carotenoids, natural products, biodiesel, or other fluids that are
above their melting point and below their boiling point at the
system pressure and temperature. Practical examples of the feed
dispersions requiring separation include but are not limited to
minerals and gangue, aqueous dispersions of micro-organisms
(microalgae, bacteria, fungi, and/or viruses), aqueous dispersions
of oil droplets and unwanted particulates, aqueous dispersions of
triglycerides, coal and unwanted materials (e.g., ash), particles
in waste-water treatment streams, and inks and/or adhesives on
paper for recycling, or combinations thereof. Micro-organisms in
the feed may be alive or dead, whole or ruptured. The apparatus and
process of this invention is especially useful for the
concentration (dewatering) of ruptured microalgal cells and
microalgal cellular components in water. Additives may be
introduced to facilitate flotation of micro-organism cells such as
alum, ferric chloride, poly-electrolytes, polymers, and other
flocculants known in the art. The carrying liquid may be water,
brine, seawater, aqueous solutions, growing media for the
microalgae or reagents or a combination of any of these.
[0066] In biochemical process engineering, adsorptive bubble
separation finds utility in isolation or concentration of valuable
natural products such as are produced by, for example, microalgae.
Often in such applications the desired organism or biochemical
product is present in very low concentration. In such cases it is
necessary therefore to feed large volumes of a very dilute aqueous
dispersion of the desired material. See for example, "Harvesting of
Algae by Froth Flotation," G. V. Levin, et al., Applied and
Environmental Microbiology, volume 10, pages 169-175 (1962). U.S.
Pat. Nos. 5,776,349 and 5,951,875, the contents of which are
incorporated herein by this reference, disclose the use of a
Jameson cell for dewatering an aqueous dispersion of ruptured
microalgae cells.
[0067] The microalgae can be any species of microalgae one desires
to separate from the carrying liquid. These species include, but
are not limited to Anabaena, Ankistrodesmus falcatus, Arthrospira
(Spirulina) obliquus, Arthrospira (Spirulina) platensis,
Botryococcus braunii, Chaetoceros gracilis, Chlamydomonas
reinhardtii, Chlorella vulgaris, Chlorella pyrenoidosa,
Chlorococcum littorale, Cyclotella cryptica, Dunaliella bardawil,
Dunaliella salina, Dunaliella tertiolecta, Dunaliella viridis,
Euglena gracilis, Haematococcus pluvialis, Isochrysis galbana,
Nannochloris, Nannochloropsis sauna, Navicula saprophila,
Neochloris oleoabundans, Nitzschia laevis, Nitzschia alba,
Nitzschia communis, Nitzschia paleacea, Nitzschia closterium,
Nostoc commune, Nostoc flagellaforme, Pleurochrysis carterae,
Porphyridium cruentum, Prymnesium, Pseudochoricystis ellipsoidea,
Scenedesmus obliquus, Scenedesmus quadricauda, Scenedesmus acutus,
Scenedesmus dimorphus, Skeletonema costatum, Spirogyra, Spirulina,
Synechoccus, Amphora, Fragilaria, Schizochytrium, Rhodomonas, and
genetically-engineered varieties of these microalgal species. It
should be understood that an additional reason for the separation
of microalgae may be to clean the carrying liquid, rather than just
for the purpose of concentrating microalgal biomass.
[0068] In mineral processing applications, the feed dispersion is
conditioned using surface chemistry treatments as are known in the
art that render the desired mineral hydrophobic. When the feed
dispersion is contacted with gas, the hydrophobic materials attach
and rise with the bubbles. The undesired gangue material then flows
downward with the liquid. This invention is also useful in those
instances where the undesired material is rendered hydrophobic and
is removed with the froth. In such cases the underflow contains the
desired material.
[0069] Due to the difficulty in transporting froths in ducts and
pipes, it is critical to collapse the purified froth in the
collection launder. The purified froth which overflows into the
collection launder naturally collapses or is treated in one of
several ways known in the art to collapse the froth and isolate the
concentrated material. With more persistent froths, more aggressive
action must be taken to collapse them. The use of froth sprays in
the froth collection launder is common in the prior art. The liquid
used for the sprays can be water or any other liquid. So as to not
dilute the collected material, the liquid portion of the collapsed
froth can be recirculated through the spray nozzles. It is
important that the spray nozzles be of a design to prevent
clogging. Air or gas selected from those used for creation of the
foam can also be used to break the froth.
[0070] Persistent froths (which do not collapse easily) are
typically broken down by the use of sprays of liquid into the
collection launders after the froth has overflowed into them. When
the froth is allowed to flow to the perimeter of the separation
vessel for collection in launders, the area needing treatment is
the entire perimeter of the vessel. This addition of spray liquid
can be large thus diluting the froth and partially defeating the
purpose of the operation. This dilution is undesirable because
adsorptive bubble separation is commonly used for the concentration
of a hydrophobic material. Therefore, the instant invention
provides an improvement in that the central collection launder has
a lower surface area and requires less spray volume.
[0071] Chemical methods for breaking froth are also known in the
prior art and include, but are not limited to the use of chemical
defoamers. These defoamer liquids can be sprayed on the froth
surface or distributed within a wetted pad that contacts the froth
as it flows into the froth collection launder. These chemical
methods are also applicable for use in the instant invention and
the lower surface area of the central collection launder may result
in the use of less chemical defoamer.
[0072] Mechanical methods to break the froth include, but are not
limited to sonication methods, vibrating or spinning objects in the
froth region, etc. Also, combinations of chemical and mechanical
coalescing techniques can be used to coalesce the bubbles and form
a region enriched in the particles to be separated from the liquid
stream. These mechanical methods are also applicable to use in the
instant invention and the lower surface area of the central
collection launder may use smaller and less expensive
equipment.
[0073] Described are improved froth flotation separation processes
that use partial vacuum (i.e., suction or downdraft) to pull the
froth and/or collapsed froth into and through the froth collection
launder and froth drain line. This improvement greatly assists
froth collection (especially with persistent froths) because froth
can be pulled by suction through the collection system more easily
than it can be drained by gravity or pushed. This partial vacuum or
suction can be generated by any method known in the art, but it is
particularly useful if it is created as a result of the need to
supply gas to generate the gas-liquid-particle dispersion. By
supplying the gas for the gas-liquid-particle production from the
head space of a tank receiving the collected froth and collapsed
froth, the partial vacuum or suction is created. This vacuum-aided
froth collection embodiment is illustrated in FIG. 10 in
integration with the apparatus previously described in FIG. 2 but
it may be used in any flotation cell using a froth collection
launder.
[0074] The vacuum source for this optional performance enhancement
may be generated by the gas blower or compressor 84 which provides
the flotation gas 85 either for sparging into the flotation cell or
for mixing with the liquid-particle dispersion 86 to give the
gas-liquid-particle dispersion 87. This may be achieved by the use
of a collapsed froth trap 88 from which the gas blower or
compressor 84 obtains its supply of gas 89 via supply line 90.
Alternatively the vacuum can be self-generated by the use of an
aspirating aerator (e.g., aspirator, venturi or eductor) 91 which
is used to create the gas-liquid-particle dispersion 87. In the
case of self aspirating aerators, no gas blower or compressor 84 is
needed but the vacuum is provided by the Venturi effect of the
aspirating aerator 91. Other benefits of this vacuum-aided froth
collection embodiment are the pre-saturation of the flotation gas
with the liquid in use and the ability to use a closed flotation
cell with an inert gas.
[0075] A second optional enhancement, spray-aided froth collection,
illustrated in FIG. 10, may be used with or without the previous
vacuum-aided froth collection. In this optional enhancement, a
spray is used to improve froth collection. Pump 92 is used to
generate the spray 93 from collapsed froth liquids 94 in trap 88.
The spray 93 drives the persistent froth into the froth collection
launder 95. The use of collapsed froth liquids for this spray
prevents dilution of the collected materials.
[0076] In the case of particulate flotation, where the gangue
material is denser than the liquid in the feed dispersion, a sloped
bottom and a solids relief discharge will be needed to remove
solids from the separation vessel. If the feed rate of the feed
dispersion is somewhat constant, this solids relief discharge can
be controlled by the use of a small valve set to discharge from the
sloped bottom and remove the solids in a heavy slurry. In another
aspect, this removal could be through a small solids handling
pump.
[0077] Any suitable liquid can be used for the optional froth
washing operation. Suitable liquids include, but are not limited to
water, liquids that are native to the feed dispersion, solutions of
surface treatment and conditioning agents, and combinations
thereof.
[0078] FIG. 11 illustrates the froth crowding effect that occurs as
the floating froth travels from a liquid surface region where it
forms to a liquid surface region of lower area before spilling into
the collection launder. This top view of a cylindrical flotation
cell 96 of the type shown in FIGS. 2 and 3 emphasizes how the froth
in, for example, region 97 must crowd into region 98 then region 99
before spilling into the central froth collection launder 100.
Imaginary radial lines 101 and 102 and equi-distant concentric
circles 103 and 104 are shown for illustration purposes only.
[0079] The process and apparatus for adsorptive bubble separation
is further illustrated by the following Examples.
EXAMPLES
Example 1
Extractant Recovery From Copper Extraction Raffinate
[0080] A pilot-scale flotation cell is constructed with the design
shown in FIG. 5 with the overall dimensions of the cylindrical
separation zone section being 0.3 meters diameter and 1.3 meters in
height. The annular clearance between the froth baffle and the
outside is 1.25 centimeters and the froth collection launder lip
diameter is 11.5 centimeters. The separation zone is fed through a
5 centimeter diameter introduction duct with an aqueous feed
dispersion containing 0.02 wt % kerosene (solvent) with 0.005 wt %
of Cyanex extractant. The feed dispersion rate is 35 liters per
minute and the gas (air) flow rate is 8 liters per minute. The Jg
in the separation zone is 0.62 centimeters/second, and this shows a
moderate transport rate of collected material from the separation
zone into the froth zone. The bubble-particle agglomerates
residence time in the separation zone is 168 seconds, while the
froth residence time in the froth zone is 562 seconds. Ninety
percent of the kerosene fed to the flotation cell is recovered in
the froth.
Example 2
Extractant Recovery From Copper Extraction Raffinate
[0081] A pilot-scale flotation cell is constructed with the design
shown in FIG. 5 with the overall dimensions of the cylindrical
separation zone being 0.3 meters diameter and 1.3 meters in height.
The annular clearance between the froth baffle and the outside is
1.25 centimeters and the froth collection launder lip diameter is
8.9 centimeters. The separation zone is fed through a 7.5
centimeter diameter duct with an aqueous feed dispersion containing
0.02 wt % kerosene with 0.005 wt % Cyanex extractant. The feed rate
is 53 liters per minute and the gas (air) flow rate is 13 liters
per minute. The Jg in the separation zone is 1 centimeter/second,
and this shows a high transport rate of collected material from the
separation zone into the froth zone. The bubble-particle
agglomerates residence time in the separation zone is 96 seconds,
while the froth residence time in the froth zone is 321 seconds.
Eighty-five percent of the kerosene fed to the flotation cell is
recovered in the froth.
Example 3
Dewatering of an Algal Dispersion
[0082] A commercial-size flotation cell is constructed with the
design shown in FIG. 5 with the overall dimensions of the
cylindrical separation zone being 1.25 meters diameter and 2.5
meters in height. The annular clearance between the lower froth
baffle and the outside wall is 0.125 meters and the froth lip is
0.58 meters from the upper froth baffle. The separation zone is fed
through a 0.2 meter diameter duct with a gas-aqueous dispersion
containing 0.05 wt % ruptured Dunaliella salina microalgae. The
feed rate is 750 liters per minute, and the gas flow rate is 190
liters per minute. The Jg in the separation zone is 0.71
centimeters/second, which shows a high transport rate of collected
material from the separation zone into the froth zone. The
bubble-particle agglomerates residence time in the separation zone
is 117 seconds, while the froth residence time in the froth zone is
390 seconds. The collected froth after collapsing shows a
twenty-fold increase in ruptured microalgae concentration.
Example 4
Mineral Recovery
[0083] The same pilot-scale flotation cell described in Example 3
is used herein. A 25 wt % aqueous slurry of copper sulfide ore
chalcocite (Cu.sub.2S) particles with a d80 particle size of 80
microns (i.e., 80% of the particles are less than 80 microns) is
fed to the cylindrical separation zone from the central duct. The
density of the copper sulfide ore particles is 5.5 g/ml, and the
solids contain 0.1% chalcocite. The unfloated solids are discharged
through a solids relief line at a solids flow velocity greater than
4 feet/second in order to prevent deposition of the solids and
sanding of the line. Thus a high percentage of the total liquid
underflow flows through the solids relief valve. The mean residence
time of the chalcocite particles in the separation zone is 156
seconds, and the mean residence time of the chalcocite particles in
the froth zone is 520 seconds. The chalcocite particles exit in the
froth collection launder with 2 wt % chalcocite and a single-pass
recovery rate of 65%.
Example 5
Dewatering of an Unruptured Microalgal Dispersion
[0084] A commercial-size flotation cell as per Example 3 is
constructed. The separation zone is fed through a 0.2 meter
diameter duct with an gas-aqueous dispersion containing 0.05 wt %
of whole Dunaliella salina microalgae together with alum and
polymer as is known in the art. The liquid feed rate is 750 liters
per minute, and the gas flow rate is 190 liters per minute. The Jg
in the separation zone is 0.71 centimeters/second, showing a high
transport rate of collected material from the separation zone into
the froth zone. The bubble-particle agglomerate residence time in
the separation zone is 117 seconds, while the froth residence time
in the froth zone is 390 seconds. The collected froth (after
collapsing) shows a twenty-fold increase in microalgae
concentration.
* * * * *