Note: Descriptions are shown in the official language in which they were submitted.
~055168
This invention relates to a method and an apparatus for
the separation of solid particles from a liquid, such as
waste water or sewage water. The particles are removed from
the liquid with the aid of microbubbles, minute gas bubbles,
clinging to the particles and carrying them upward to the
liquid surface.
It is previously known to generate microbubbles in a
first liquid by causing a pressurized second liquid which has
a gas dissolved in it, whose amount is relatively large because of
the pressure, to flow into the first liquid. A great many micro-
bubbles are formed at the expansion of the second liquid in the
first liquid and at the release of the gas dissolved under
pressure. The microbubbles rise toward the surface of the first
liquid and when colliding with each other they gather together
into large bubbles. The frequency of the gathering together of
the microbubbles is dependent upon the number of the bubbles
per volume unit. By reason of this gathering together a dense
stream of microbubbles will also contain many large bubbles
when it reaches the liquid surface. However, the tendency of
the microbubbles to gather together and thus their ability of
clinging to particles in the liquid also grows with the de-
creasing size of the bubbles because the internal pressure of
the bubbles will increase as their size decreases.
Microbubbles generated in the manner outlined above are
utilized for the separation of small particles and suspended
1055168
particles as well as colloidal particles from a liquid. When
the microbubbles come near the particles they are attracted to
them with a force which is substantially inversely proportional
to the bubble size, and the bubbles form agglomerates with the
particles. When bubbles in sufficiently large number have clung
to a certain particle in the li~uid - the requisite number is
dependent upon the common volume of the different bubbles and
the weight and the specific surface of the particle - the agglo-
merate formed will be so lightweight in relation to the surround-
ing li~uid that it begins to rise toward the surface of theliquid. By means of a large quantity of microbubbles generated
as outlined above in a liquid containing suspended and colloidal
particles, it is possible effectively to separate the particles
from the liquid even though there is a relatively large quantity
of particles.
Chemists have developed sundry processes of reducing the
tendency of the microbubbles to be attracted to each other
and to gather together, in order to exploit them better in a
microflotation system for the separation of suspended and
colloidal particles therefrom. In an early stage of, and often
before the microbubble generation, they have studied what effect
the addition of certain agents such as, on the one hand, electro- -
lytical substances, coagulants and flocculating agents and,
on the other hand, relatively large aggregate particles or
flocs have on the freauency at which bubbles gather together.
Certain additives contribute to binding the microbubbles to the
aggregate particles and thereby prevent a higher frequency
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of microbubbles gathering together. The particles to be separated
from the liquid cling in their turn to the aggregate particles
which are carried to the surface of the liquid by the bound
microbubbles. This technique has been successful to some degree,
but the separating capacity of microflotation systems operating
on this principle is unsatisfactorily low. Moreover, an ex-
pensive dosage equipment is required for the addition of electro-
lytical substances, coagulants, surfactants and flocculating
agents.
One object of the present invention is to provide a higher
separating capacity at the microflotation of particle-laden
liquids.
Another object of the invention is simply and effectively
to prevent an early gathering together of the minute gas
bubbles, so-called microbubbles, which are formed when the
pressure liquid is caused to expand and flow into the liquid
from which suspended and colloidal particles are to be separated.
Quite surprisingly, it has now been found according to the
present invention that the separating capacity of microflotation
2- systems can be considerably increased and that this increase -
can be attained without any surface-active additives, although
small amounts of surfactants added to the flotation liquid are
preferred. According to the invention it is possible to prevent, -
by simple and inexpensive means, that the microbubbles at their
generation and immediately thereafter gather together into
large bubbles which are less effective or not effective at all
as flotation agents.
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105516~
According to a preferred embodiment of the present invention
it has been found that the separating capacity of the micro-
flotation system can be checked by a control of the residence
time of the flotation liquid in the system, which can be regu-
lated within broad limits.
The present invention relates to a method and an apparatus
whereby an improved separating capacity is attained at the
microflotation of colloidal particles and particles suspended
in a raw phase for the preparation of a clear phase which to
a surprisingly high degree has been freed from the particles.
The raw phase can be a contaminated liquid, for instance
different types of waste water, but many other types of raw
phases containing suspended particles, such as fibres and
crystals and/or colloidal particles, can be purified in this
manner, for instance such raw phasesas occur in the metallur-
gical industry.
The raw phase, that is, a liquid containing particles, -
is pumped into a container or tank. Another liquid containing
gas dissolved under pressure, that is, a pressure liquid, is
caused to expand and flow into the raw phase so that minute
bubbles, so-called microbubbles in the size of 10 4 - 10 3 mm, of
the dissolved gas are formed in the raw phase. The microbubbles
cling or adhere to the suspended and colloidal particles of the
raw phase. This adhesion is facilitated if the gas bubbles remain
minute and do not gather into large bubbles. I~hen the particles --
have united with a given number of microbubbles they become more
lightweight than the ambient raw phase liquid and thus begin to
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lOSS168
rise toward the surface of the raw phase liquid.
Particularly characteristic of the method according to the
invention is that a dynamic pressure increase is imparted to
the pressure liquid immediately before the expansion of the
liquid, and immediately when expansion begins a rapid heavily
diverging stream is spread into the raw phase. The microbubble
formation is postponed for a short time after the expansion by
reason of the high pressure and only starts when the pressure
liquid has become well distributed in the raw phase. The effect
is that the gathering together of microbubbles into large
bubbles will at least initially be almost completely prevented
and that there will be a both high and homogeneous concentra-
tion of microbubbles in the raw phase, which implies a high
separation and flotation capacity of the system, as has been
pointed out above.
The flotation capacity can be still more improved in that
the diverging flow of pressure liquid is used for liquid jet ~-
pumping of the raw phase so that the raw phase is pumped upward
in a first chamber of the flotation tank during the flotating
and separating operation and is discharged from said chamber
when it has been freed from particles (clear phase) to a
second chamber in the tank for the separation of the particle
froth which has formed on the surface of the clear phase. The
first chamber is so designed and dimensioned in view of the
desired diverging flow of the pressure liquid that a substan-
tially laminar and non-turbulent raw phase stream can be attained
in this chamber during the flotation procedure.
1055168
In a preferred embodiment of the invention, untreated raw
phase is supplied to the system at the inlet of the first chamber
below the diverging flow. The non-treated raw phase is thereby
raised through the diverging stream and comes in contact with
a particularly high and homogeneous concentration of particular-
ly small and "active" microbubbles before it is further raised
toward the liquid surface by the liquid jet pumping. The flo-
tation process of the present invention is so effective that
additions of surfactants, such as electrolytical substances,
coagulants and flocculents can be dispensed with. It may,
however, be worth while to add relatively small amounts of
such substances. In accordance with another preferred embodiment
of the invention these substances should then be uniformly dosed
into the diverging stream of pressure liquid or into the pressure
liquid itself immediately before the pressure increase.
In another preferred embodiment of the invention, the system
is continuous in the sense that an optional portion of the
clear phase is recycled to the first chamber by adjustment of the
entering flow of raw phase and is mixed at the inlet with the --
supplied non-treated raw phase ~elow the diverging flow. In
this way the purity of the clear phase can be considerably in- ~ -
creased simultaneously as the system is still compact. With a 1 -
recycled clear phase residence times and cycling conditions in
the system are easily regulated. With an increase of the pressure
liquid flow it is possible also to increase the circulation in
the first and second chambers without it being necessary to
change the raw phase supply and clear phase withdrawal to any
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~055168
greater extent, which means that the separation capacity of the
system and the purity of the clear phase can be finely adjusted.
An increased flow of pressure liquid in fact results in an in-
creased amount of microbubbles per volume unit and time unit
in the first chamber.
The method of the invention can be carried out in practice
by means of an apparatus comprising a tank having first and
second chambers which are separated by a wall terminating near
the liquid surface in the tank. Means are provided in the se-
cond chamber for removing from the liquid surface froth whichcontains suspended and colloidal particles and microbubbles
clinging thereto as well as additives, if any. Further means
are provided in the second chamber at a lower level in the
tank for withdrawal of the clear phase which has flown over
from the top region of the first chamber.
The feature specifically characteristic of the apparatus
according to the invention is a nozzle (or possibly more nozzles~, ~
said nozzle being spaced some distance from the bottom of the -
first chamber. The nozzle is so designed as to impart to the
pressure liquid an excess pressure immediately before the ex-
pansion and to produce at the expansion a heavily diverging
flow of the pressure liquid into the raw phase. The minute gas
bubbles formed a very short time after the expansion are rapidly
spread over the entire horizontal cross-section of the first
chamber and the frequency at which bubbles gather together will
be very low. Such a nozzle is also advantageous in that during
the particle flotation by means of the gas bubbles from the nozzle
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1055168
by liquid jet pumping the raw phase is raised through the first
chamber. When bein~ highly freed from particles (clear phase) the
raw phase can flow down as a surface layer into the second
chamber in order to be removed therefrom in a suitable manner.
According to a preferred embodiment, the nozzle or nozzles
shall have such a configuration that the flow of pressure liquid
will be in the shape of a cone, preferably a whole cone, having
a top angle of 30-150. Further, the nozzle shall be so dimen-
sioned and the pressure of the pressure liquid at the expansion
shall be such that the conical flow well fills out the first
chamber which in that case suitably is of a cylindrical shape.
The liquid jet pumping generated by the pressure liquid flow
will thus be able to lift the raw phase upward through the
first chamber in a substantially laminar and non-turbulent
stream.
In another preferred embodiment of the invention the
first chamber may be a cylindrical shell and the second chamber
may be that cylinder which has the inner wall of the cylindrical
shell as a boundary surface. The above-described nozzle should
then suitably be placed over the bottom of the second chamber,
but have a pressure liquid flow in the form of a cone with a
central conical hole so that a homogeneous laminar and non-
turbulent liquid jet pumping in an upward direction can be
maintained in the cylindrical shell, while the downward flow
of the clear phase in the cylinder chamber will be unobstructed.
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1055168
In accordance with the invention, it is also conceivable
to have a flotation tank of prismatic shape. The nozzle or
nozzles should then generate in the first chamber a pressure
liquid flow of substantially rectangular cross-section. Such a
flow can be realized by means of a nozzle formed with a slot
of specific design.
Nozzles useful for the purpose of the present invention are
manufactured int. al. by Spraying Systems ~ompany. Their ver-
sions of Full Jet Nozzle and Whirljet Noæzle of varying volume
capacity give the preferred rapid whole cone and hollow cone
flows necessary for a high separation technique in a microflo-
tation system according to the invention.
~ The invention will be described hereinbelow with reference
to the accompanying drawing which illustrates a microflotation
tank according to the invention in axial section.
Liquid containing suspended and colloidal particles (raw
phase) is supplied to the cylinder shell shaped chamber 1 through
the pipe 2 and is mixed with liquid which has been freed from
particles (clear phase) and flows downward through the chamber 1.
The mixed liquid material flows through the openings 3 into the
bottom region of the cylindrical chamber 4. It is sucked upward
by the li~uid jet pumping generated by the whole-cone pressure
liquid stream from the nozzle 5 and then flows, after surfactants
have possibly been added via the device 6, further upward to
the liquid surface in the chamber 4.
The pressure liquid contains dissoIved air and when the
liquid, after a pressure increase immediately before the expan-
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~055168
sion, rapidly expands into the mixed raw phase there is almost
immediately formed a great many minute air bubbles in the raw
phase. By reason of the pronounced conical shape and the high
speed of the pressure liquid stream the air bubbles are kept se-
parated in spite of their tendency of gathering together. They
now have good possibilities of uniting with suspended and colloi-
dal particles since they are still small but many in number and
homogeneously divided and are attracted to these particles with
great force. The particles change in appearance when the air
bubbles cling to them, become thicker and lighter, rising toward
the liquid surface in the tank. The liquid jet pumping accele-
rates the rise of the particles. The flotated particles finally
enter into the froth phase at the surface of the liquid freed
from particles.
The liquid is now highly freed from the suspended and
colloidal particles when it flows over into and down through
the cylinder shell shaped chamber 1. Part of the clear phase
is removed through the pipe 7 while the remainder is recycled
to the system through the openings 3. The froth layer formed
on the clear phase is removed through the chute 8.
The various phases of the flotation process can be divided
as follows into zones:
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1055168
a~ The inner cylinder, riser~
-
I. Expansion, bubhle formation and possible addition of
chemicals such as coagulants and flocculents.
II. Mixing of raw phase, clear phase and microbubbles and
initial union between particles and microbubbles.
III. Continued union between particles and bubbles, floccula-
tion and initial separation of large agglomerates of
particles.
IV. Continued flocculation and separation.
V. Froth formation and phase separation.
b) Cylinder shell
V. Froth withdrawal, downward traveling clear phase.
IV. Downward traveling clear phase.
III. Withdrawal of clear phase.
II. Introduction of raw phase and mixing with recycled clear
phase.
I. Flow into inner cylinder.
Example 1
1 m of raw phase consisting of a cellulose fibre suspen-
sion in water was microflotated in a tank in a conventional man-
ner but without addition of any surfactant or other substance
promoting the microflotation. Pressure water containing dis-
solved air was allowed to expand and flow into the fibre sus-
pension in a known Manner. The pressure was 5 atm. and the
volume 0.1 l/min. The suspension was not circulated and the
treatment time only amounted to 5 min. The turbidity of the
suspension was measured according to Jackson after finished
treat~ent. A considerable fibrous turbidity remainded in the
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~055~68
water,
The microflotation was then carrièd out according to the
present invention in the manner described above but with the
use of a nozzle spreading the pressure water in a heavily di-
verging rapid stream. A turbidity measurement according to
Jackson after 5 min. showed that the turbidity had been reduced
by about 90% compared to conventional microflotation.
_~ample 2
Microflotation was carried out according to the present
invention with a raw phase consisting of a liquid heavily
clouded by soap residues. The turbidity reduction amounted to
about 50% with the use of the diverging rapid pressure water
flow according to the invention.
Example 3
At microflotation effected according to the invention on
ordinary sludge the turbidity reduction amounted to about 40~.
Example 4
Microflotation of the same sludge as in Example 3 was made
in conventional manner and with lauryl sulphate as a surface-
active addition promoting the microflotation. After 5 min. theturbidity was about 30% lower than without the use of lauryl
sulphate. When the microflotation was realized with the same
addition of iauryl sulphate and also with a diverging pressure
water flow according to the invention a turbidity reduction
of about 80% was attained.
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lOSS~68
The above embodiment of the invention can be modified and
varied in many ways, for instance as outlined in the foregoing,
within the spirit and scope of the invention.
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