CA1055168A - Method and apparatus for microflotation by means of pressure liquid - Google Patents

Abstract

METHOD AND APPARATUS FOR MICROFLOTATION BY MEANS OF PRESSURE
LIQUID

Abstract of the Disclosure This invention relates to flotation treatment of a first liquid in a container for cleaning said first liquid from con-taminating particles by upward injection into said first liquid of a pressurized second liquid containing gas dissolved therein under pressure, whereby dissolved gas will be liberated from said second liquid by the expansion thereof in said first liquid and spread as minute bubbles into said first contaminated liquid to entrain and carry contaminating particles to the surface of the liquid in the container, wherein said second liquid is sub-jected to a dynamic pressure increase immediately before the expansion thereof and is spread upward into the first conta-minated liquid in a substantial divergent, homogeneous fast flowing flow for effectuating the initiation of the generation of gas bubbles only when the injected second liquid is well distributed in the contaminated liquid.

Description

~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 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 ''''' ' 1.

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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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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 . , .

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.

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 ' . , ., : , ., ' , .. . .

~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 '' 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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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-.... 10 .. , '' , ,' ', ", . .....
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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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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 ~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.

, 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.

Claims (14)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of flotation for the treatment of a first liquid in a container, said first liquid in said container con-taining suspended and/or colloidal particles, and representing a raw phase to be treated, a second liquid having gas dissolved under pressure therein being caused to expand and flow into the first liquid for liberating gas dissolved under pressure in the second liquid, and generating minute gas bubbles, micro-bubbles, which will tend to entrain said particles and raise them to the surface of the first liquid, wherein said second liquid is subjected to a dynamic pressure increase immediately before the expansion thereof and is spread upward into the first contaminated liquid in a substantial divergent, homogeneous fast flowing flow for effectuating the initiation of the gene-ration of gas bubbles only when the injected second liquid is well distributed in the contaminated liquid.

2. A method as claimed in claim 1, wherein the divergent flow is directed upward in a first chamber of the container for realizing in said chamber a liquid jet pumping action for accelerating the transport and separation of the entrained par-ticles and the entraining microbubbles from the first liquid, and wherein the froth formed by said particles and microbubbles and a clear phase of said first liquid are separately removed from a second chamber of the container.

3. A method as claimed in claim 2, wherein untreated raw phase as said first liquid is pumped into the container below the region for the divergent flow and is raised by said flow, the raising effect of which is enhanced by said jet pumping action, and thereby mixed with the microbubbles spread in said flow.

4. A method as claimed in claim 3, wherein part of the clear phase is received in a second chamber and is recycled to said first chamber and mixed with the untreated raw phase while it flows in said divergent flow.

5. A method as claimed in claim 1, wherein additives are dispensed into the divergent flow or into the entering raw phase.

6. A method as claimed in claim 1, wherein the first liquid is water.

7. A method as claimed in claim 1, wherein the first liquid is an organic solvent.

8. A method as claimed in claim 1, wherein the particles are fibres.

9. A method as claimed in claim 1, wherein the particles are of a mineral nature.

10. A method as claimed in claim 2, wherein the treatment time and the recycling conditions are controlled by adjusting both the raw phase and clear phase flows and the pressure liquid flow.

11. An apparatus comprising a container for flotation treatment of a particle-laden first liquid which constitutes a raw phase, in which a pressurised second liquid which contains gas dissolved under pressure is caused to expand in the raw phase for liberating gas as minute gas bubbles, microbubbles, therein and for entraining particles by said gas bubbles thereby raising said particles to the liquid surface, said container having first and second chambers and a wall separating the cham-bers and terminating at the liquid surface, and means in the second chamber for removing from the liquid surface froth con-taining particles and microbubbles and for removing treated liquid in the form of a clear phase, that is, liquid freed from particles, and further comprising nozzle means disposed above the bottom of the first chamber to impart to the pressure liquid a dynamic pressure increase immediately before the expansion of the liquid and for generating a substantial divergent fast flow into the raw phase so that said generated microbubbles will be spread rapidly over the entire horizontal cross-section of said first chamber and so that the raw phase is raised by said liquid jet pumping action upward through the first chamber.

12. An apparatus as claimed in claim 11, wherein the nozzle means is adapted to provide a conically divergent flow with a cone angle of 30-150° and the first chamber is substantially cylindrical for maintaining a highly laminar non-turbulent liquid jet pumping action in the first chamber.

13. An apparatus as claimed in claim 11, wherein said first chamber has an annular cross-section coaxially encircling said second chamber which has a cylindrical cross section, wherein the nozzle means is adapted to generate a flow having the form of a hollow cone and wherein the two chambers are so arranged with regard to the dimensions of said hollow cone that the pressure liquid flows exclusively into the first chamber for providing in said first chamber a highly laminar and uniform pumping of the raw phase upward.

14. An apparatus as claimed in claim 11, wherein said first chamber is of prismatic shape and the nozzle means has a slot-shaped outlet opening for providing a pressure liquid outlet flow of rectangular cross-section.