CA1096515A

CA1096515A - Process and installation for eliminating by flotation impurities in the form of solid particles contained in a liquid

Info

Publication number: CA1096515A
Application number: CA275,438A
Authority: CA
Inventors: Enar V. Hellberg
Original assignee: Swemac SA
Current assignee: Swemac SA
Priority date: 1976-04-12
Filing date: 1977-04-04
Publication date: 1981-02-24
Also published as: NL180076C; FR2544225B1; CH614636A5; BR7702266A; GB1570345A; DE2760246C2; ATA247877A; FR2544225A1; FR2347984B1; JPS6057903B2; NL7703627A; DE2712947C2; FI68093C; FI68093B; AT358499B; SE8202540L; NL8601174A; SE425145B; JPS56161849A; IT1073184B

Abstract

ABSTRACT OF THE DISCLOSURE

The invention relates to a process and to an instal-lation for the elimination by flotation of impunities in the form of solid particles contained in a liquid. The installation com-prises at least one flotation cell through which pass a flow of the said liquid to be purified and a stream of air, in such a manner as to form floating foams originating from the fixation of air bubbles on the impurities; the installation further com-prises, upstream of the flotation cell, at least one narrow mixing chamber capable of permitting the passage of a thin layer of liquid to be purified and provided with a means capable of admitting the flow of air transversely of the layer of liquid over a path in the chamber.

Description

~!lJ9~S~L~5 The present invention relates to a process for the elimination by flotation of impurities which are in the form of solid particles contained in a liquid, in which process a stream of air is injected into a flow of the liquid in question in a flo-tation cell, in such a manner as to form air bubbles which aredistributed in the liquid, are fixed on the impurities, and rise to the surface so as to form a foam which is charged with these impurities and which is discharged from the surface of the liquid.
A flotation process of the kind described above is 10 known for the removal of ink from paper pulp prepared from old papers, according to which air is introduced directly into the bottom of the cell by suction with the aid of a propeller which also serves for the intimate mixing and distribution of the fibrous suspension in the cell.
The admission of air directly into the pulp undoubtedly makes it possible to regulate the air pressure and therefore the diameter of the bubbles in dependence on the volume of air, which in turn is adjustable within certain limits. Although an increase of the volume of air makes it possible to raise the pressure and consequently increase th diameter of the air bubbles, by this known process it is on the other hand impossible to reduce the diameter of the bubbles while increasing the flow of air, which in certain cases may be necessary in order to effect the separation of certain impurities~
It has in fact surprisingly been found that it was possible to effect selective separation of the impurities by 10ta-kion, by controlling the dimension of the bubbles and the volume~
of air independently of one another. Thus, for example in the case of the de-inking of defibred, pretreated paper pulp prepared from old papers, it is now possible to obtain diferent bubble dimensions -1 for the flotation of the above inks, for the flotation of fillers such as kaolin, or for the flotation of fibres, and thus to proceed to their selective separation.
The present invention has in particular as its object a process permitting the adjustment of the dimensions of bubbles independently of the volume of air mixed with the water which is to be purified.
To this end, according to the invention, before in-jecting the air and the liquid into the flotation cell, this liquid is circulated in at least one mixing chamber in the form of a thin layer, preferably 2.5 - 4 mm thick, while the air is simul-taneously admitted into this layer of liquid transversely of the path of the latter and along a portion of this path.
In cases where the liquid is a suspension of fibres, such as paper pulp, it has in addition been found that the forced passage of the pulp in the form of a thin layer in a narrow chamber and its encounter with divided air injected transversely onto the fine layer of liquid charged with suspended fibres give rise to an effect of rubbing and cleaning these fibres.
According to one characteristic of the process of the invention the thickness of the layer of liquid in the chamber and consequently the speed of passage of the liquid in the chamber are -`
regulated.
In the case of the de-inking of pulp made from old papers, it has been found that the volume of air mixed with the pulp and the speed of passage of the pulp in the mixing chamber have an effect on the whiteness of the pulp, and that by adjust-ing them it is possible to obtain differences of whiteness ranging from 57 to 66.5 Scan.
These adjustments make it possible to achieve effective purification with minimum rejection.
According to another characteristic of the process, the length of the path of admission of air into the liquid is regulated.
It is thus possible for the amount of air to be accu-- -rately proportioned in relation to the layer of liquid passing through the mixiny chamber.
Yet another characteristic of the process consists in entraining the foams by a ~low of air into a hopper and recovering the air for recycling.
In the case of flotation with a plurality of cells, a mixture of air and liquid coming in each case from a mixing chamber is admitted into each cell. It is thus possible to control the dimensions of the bubbles differently from one cell to another, in order to recover the impurities selectively When the liquid contains fibres, in order to avoid maximum loss of the latter, it is possible for the foams coming from other cells to be recycled to the liquid to be purified which passes into the first flotation cell.
The present invention also relates to an installation for the application of the process.
According to the invention the installation also com-prises, upstream of the flotation cell, at least one narrow mixing chamber capable of allowing the passage of a thin layer of liquid to be purified and provided with means capable of admitting the flow of air transversely of the layer of liquid over a path in the chamber According to one characteristic the mixing chamber is bounded by two parallel walls at a distance from one another, so as to allow the passage of liquid to be purified Accordiny to another characteristic the mi~ing chamber s~

is provided with a means of regulating the distance between the parallel walls.
Other features and details of the invention will be seen from the description of an installation given with reference to the drawings accompanying the present specification and showing, solely by way of example, one embodiment of the invention which is suitable in particular for the de-inking of pulp prepared from old papers.
Figure 1 is a diagrammatical view in elevation of a multi-storey installation;
Figure 2 is a horizontal section on the line II-II in Figure 1;
Figure 3 shows on a larger scala a view in longitudinal vertical section of a mixing chamber;
Figure 4 is a view in section of the line IV-IV in Figure 3;
Figure 5 shows on a larger scale and in vertical section ; a first form of construction of a mixing chamber;
Figure 6 is a view in section on the line VI-VI in Figure 5, and Figure 7 is a diagram showing whiteness curves.
In these drawings the same reference numerals designate identical elements.
The installation comprises four flotation cells 1,`'2, 3, 4. These cells are annular and are axially superimposed so as ko constitute ~he four stages of a tower having a plurality of floors 5. Obviously this tower could have a number of stages difEersnt from four.
The defibred, chemically pretreated paper pulp in the form of a fibrous suspension containing a foaming agent is delivered 5~S

by means of a pump 6 t'nrough a pipe 7 to the inlet of the cell 1, first passing through a mixing chamber 8. This chamber has a nar-row passage in which the fibrous suspension circulates in the form of a thin layer, while at the same time air is admitted into it transversely of the path of this layer of paste or along a portion thereof through a pipe 9 connected to an air pressure source (not shown), with the interposition of a valve 10.
The mixing chamber 8 is in communication with the annular cell 1 through a pipe 11 which leads tangentially into the cylindrical wall of the cell near its floor 5, so that the fibrous suspension is caused to turn about the central pipe 12 of the annu-lar cell. The bubbles created in the suspension are fixed on the impurities and cause them to rise to the surface in the form of ; foams. The latter are entrained out of the cell through the out-let 18 provided in the pipe 12,ppartially through the rotational , movement of the suspension and partially by a current of air provi-ded above them. A vertical plate 50 disposed above the level of the pulp guides the foams to the outlet. The foams are collected in a hopper 13 surrounding the pipe 12 and received in a reject tank 37 filled with water. The flow of air for the discharge of the foams is introduced into the upper portion of the cell through a pip~ 14 connected to a suction device 15 disposed above a frusto-conical axial pipe 16 for the recycling of the foam discharge air.
This pipe is disposed concentrically in the central foam discharge pipe 12 and is common to all the superimposed cells. The foam dis-charge air is admitted into the cell through the opening 17 and leaves it through the outlet 18, entraining the foams, which through the action of gravity fall into the hopper 13 while the air escapes through the pipe 16, The latter extends into the adjacent cell o~ming a cone narrowing in the upward direction and disposed facing the wider pipe for the recycling of the foam discharge air of this adjacent cell and of the other cells 3, 4. The parts constituting the pipe 16 form a cyclone, thus imparting a centri-fugal velocity to the foam, permitting the separation of the air which is drawn into this pipe by the action of the section device 15, whence it is returned to the pipe 14 for reuse for the dis-c'narge of foams.
The pulp partially cleaned in the cell 1 leaves th~
latter through a pipe 19 provided with a pump 20 enabliny the pressure to be increased. The pipe 19 penetrates into the mixing chamber of the next cell 4 situated in the last stage of the tower, and the process applied is identical to that utilised in the first cell, and so on in the other cells. The latter are fed success-ively with the pulp or water containing fibres of an increasing degree of purity, while the foams cGllected in these other cells are in turn recycled to the lower cell.
The foams containing the rejected material from the first cell and other cells can of course either be recylced or discharged at each stage for treatment in a separate device, depending on the type of product for which the pulp is intended.
The installation also comprises a cover 40 closing the cell of the last stage; an inspection window 41 permi~ting rapid discharge for cleaning purposes; an emptying valve 42 for each cell; a flap 46 in the foam transporting air pipe; a floor 43 in ; 25 the pipe 12 for receiving recycled rejected materials; a pipe 44 for the discharge of this rejected material in cell 1, and a pipe 45 or the discharge o~ the accepted pulp in the cell 2 treating the most highly purified pulp.
E'igures 3, 4, and 5, 6 illustrate respectively two ~orms of construction of the mixing chamber In the form of construction shown in Figures 3 and 4 the mixing chamber has a rectangular section comprising two para-1-1~ plane walls 21, 22 mounted in a parallelepipedic body 23, and forming between them a channel in communication with the pulp inlet and outlet pipes 7 and 11. The wall 21 is movable parallel to the wall 22 and is operated by means of a threaded rod 24 guided in a tapped hole 26 provided in a sleeve 27 engaged in the upper adja-cent wall of the parallelepipedic body permitting by means of a wheel 25 the adjustment o~ the distance between the parallel walls, 1~ in order to determine the thickness o-f the layer of pulp to be subjected to the action of the jet of air. The wall 21 is provided with a diaphragm ensuring the tightness of the chamber, while the wall 22 is a porous material, in the particular case considered being of fritted glass permitting the passage of the air jets onto 15 the layer of pulp passing through the chamber, It is advantageous for the mixing chamber also to be provided with a plate 28 making it possible to close off part of the surface of the passages pro--vided in the fritted plate so as to regulate the air flow admission sur~ace.
; 20 In the embodiment shown in Figures 5 and 6 the mixing chamber 8 is conical in shape and has two parallel conical walls 29, 30 spaced apart from one another to form the passage for~ the pulp through them. The pulp is fed into the chamber through a noz71e 31 leading tangentially into the chamber through the wall 25 29 in order to impart to the pulp a rotational movement while receiving the air injected through the channels 47 provided in the inner wall 30, which are in communication with an air inlet 32 connected to a compressed air source (not shown)O The inner cone 30 is adapted to slide axially in -the outer cone 2~ so as to adjust 30 the space between these walls and consequently the thickness of the d~

layer of pulp. For this purpose the inner cone, which may be rotatable, is controlled by a tube 32 which at the same time forms the air supply pipe. A conical obturator 33 disposed inside the cone 30 is mounted on two rods 34 and 35 sliding in end walls of 5 the cone 30, and makes it possible, when desired, to close off a part of the holes of the air distributor cone, in order in this way to adjust the surface admittiny the flow of air into the mixing chamber. The adjustment of the air flow independently of the speed and rate of flow of the pulp makes it possible to create 10 in the cells a type of bubble suitable for the flotation of inks, for the flotation of kaolin, or for the flotation of fibres) and thus to achieve selective separation of these components.
The invention which has been described relates to an installation for the de-inking of paper pulp. It could be used 15 for other applications, for example for the purification of ores or for the purification of white water by flotation of fibresO
Laboratory tests have made it possible to establish a diagram, which in Figure 7 shows pulp whiteness curves 48 and 49 plotted against the ratio of air volume to pulp volume and against the 20 speed of the pulp in the mixing chamber. On the abscissa is indicated the ratio of litre of air to litre of pulp, and on the ordinate the whiteness of the pulp in degrees Scan. The curve 48 shows a pulp whiteness for a pulp speed of 4O23 metres per second, while the curve 49 shows a pulp whiteness for a pulp speed of 6.35 25 metres per secondO
By adjustment of the air-pulp mixture and of the speed of the pulp in the mixing chamber it is possible to obtain difer-ences of whiteness ranging from 57 to 66.5 Scan. The tests were carried out with old papers of the "magazine" type, containing 30 about 50% of the mechanical pulp having an original whiteness of ~5~5~
the unprinted edges of 66.5 Scan.
sy applying the process of the invention the whiteness of the printed parts containing about 2% of ink can be increased from 46 to 66.5 Scan, with a retention of 20 minutes. After 2.5 minutes retention the whiteness has already passed from 46 Scan to 64.5 Scan. The best present conventional process made it possible with the same paper to obtain 62.3 Scan after 20 minutes retention and about 54 Scan after 2.5 minutes.
It is obvious that the invention i~ not limited exclu-sively to the embodiments illustrated and that many modifications can be made to the form, arrangement, and constitution of certain of the elements used in its performance, without depar~ing from the scope of the present invention, provided that these modifica-tions are not in contradiction with any of the following claims.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for the elimination by flotation of impurities which are in the form of solid particles contained in a liquid, containing at least one foaming agent, according to which a stream of air is injected into a flow of the liquid in question prior to entering at least one flotation cell in such a manner as to form air bubbles which are distributed in the liquid, which air bubbles are fixed on the impurities, and rise to the surface of the liquid so as to form a foam which is charged with these impurities and which is discharged from the surface of the liquid, and in which, before the air and the liquid are injected into the flotation cell, the liquid is circulated along a path in a mixing chamber in the form of a layer, while at the same time the air is admitted simulta-neously into this liquid layer transversely of the path and along at least one portion of that path, wherein the thickness of the layer of liquid is adjusted between two opposed parallel walls of the chamber forming a flow passage for the liquid to decrease or increase the width of the passage between them and hence the velocity of the flow of the liquid through the flow passage, thereby inversely changing the size of the bubbles formed.

2. A process according to claim 1, wherein the length of the path of admission of air into the liquid to be treated is adjustable.

3. A process according to claim 1, wherein a current of air is admitted into the flotation cell at the level of the foam in order to entrain it into a hopper in which it is caused to fall to the bottom, while the air is recovered for recycling purposes.

4. A process according to claim 1 in the case of flotation with a plurality of flotation cells, wherein a mixture of liquid and air coming from a mixing cham-ber is admitted into each cell.

5. A process according to claim 4, wherein the foam is discharged from the flotation cells into a common hopper, while the currents of discharge air are subjected to suction in order to recycle them conjointly with one another.

6. A process according to either claims 4 and 5, wherein the foam coming from other cells is recycled into the liquid passing in the first flotation cell.

7. An apparatus for the elimination by flotation of impurities which are in the form of solid particles contained in a liquid and comprising at least one flotation cell, an inlet for admitting the liquid into the cell, a mixing chamber through which the liquid passes to the inlet and is charged with air to form bubbles -therein, wherein the mixing chamber comprises opposed parellelwalls forming a flow passage for the liquid through the chamber, at least one of the walls having pores therethrough, aerating means for providing a stream of air through the pores to form bubbles in the liquid flowing through the passage, adjustment means for relatively moving the walls toward and away from each other to respectively decrease and increase the width of the passage between them and hence the velocity of the flow of the liquid through the passage, thereby inversely changing the size of bubbles formed, and inlet means for providing a flow of the liquid into the upstream end of the passage.

8. Apparatus according to claim 7, wherein said adjustment means is adjustable to provide a spacing between said walls of small depth relative to its width.

9. Apparatus according to claim 8, wherein the walls are conically shaped and coaxial, and the adjustment means moves the walls axially relative to one another.

10. Apparatus according to claim 9, wherein the inlet means is arranged to provide a tangential flow of the liquid into the passage.

11. Apparatus according to claim 9, wherein it includes means for conducting the flow of liquid away from the passage at a velocity no higher than that of the liquid flow through the passage.

12. Apparatus according to claim 9, wherein it includes means for controlling the flow rates of liquid to the inlet means and of air to the aeration means.

13. Apparatus according to claim 9, wherein it includes means adjustable to close a plurality of the pores.

14. Apparatus for elimination by flotation of solid particle impurities contained in a liquid, which comprises at least one flotation cell, an inlet for admitting the liquid into the cell, aerating means for injecting air into the liquid to form bubbles which attach to and float impurities to form a surface foam on the liquid in the cell; outlets for separately discharging said foam and cleaned liquid from said cell, and foam ejection means for directing a stream of air onto said surface foam to blow the foam toward and out its outlet.

15. Apparatus according to claim 14, wherein said cell is enclosed and said foam ejection means includes suction means and duct means for recycling the air of said air stream.

16. Apparatus according to claim 15, in which the cell is generally cylindrical, and wherein the inlet is in the lower part of the cell and is tangential to the outer wall thereof to provide a generally circular flow pattern of the incoming liquid about the cell axis, and the outlet for foam comprises an axial discharge pipe in the cell opening through the cell bottom and extending to the foam layer.

17. Apparatus according to claim 16, wherein the duct means comprises a tubular pipe disposed axially within the discharge pipe having an open end exposed to the air discharged with the foam into the discharge pipe and communicating at its opposite end with the suction means.

18. Apparatus according to claim 17, wherein the aerating means comprises a mixing chamber through which the liquid flows to the inlet and having means for injecting the air into the liquid passing therethrough.

19. Apparatus according to claim 18, wherein it comprises a plurality of cells arranged to be stacked coaxially on top of one another with the discharge pipe communicating with the cells.

20. Apparatus according to claim 19, wherein the tubular pipes are arranged to communicate when the cells are so stacked and which includes a single suction means for servicing the communicating tubular pipes and providing air streams to the foam ejection means of the cells.

21. Apparatus according to claim 20, wherein the aerating means of each cell is provided with means for regulating the bubble size produced by air injection into the liquid, and which includes piping for connecting the outler of cleaned liquid of each cell except one, to the inlet of another cell to provide series treatmetns of the liquid in the plurality of cells.

22. Apparatus for elimination of flotation of solid particle impurities contained in a liquid, which comprises at least one flotation cell, an inlet for admitting the liquid into the cell, aerating means for injecting air into the liquid to form bubbles which attach to and float impurities to form a surface foam on the liquid in the cell; outlets for separately discharging said foam and cleaned liquid from said cell, said outlet for foam comprising an axial discharge pipe in said cell opening through the cell bottom and extending to said foam outlet; and means for ejecting the foam of said layer axially inwardly through said outlet into said pipe.

23. Apparatus according to claim 22, wherein said inlet is in the lower part of said cell and is tangential to the outer wall thereof to provide a generally circular flow pattern of the incoming liquid and of the surface foam about the cell axis, and a collecting hopper for foam is provided at the lower end of said discharge pipe.