GB1570345A

GB1570345A - Ies in the form of solid particles contained in a liquid process and apparatus for eliminating by flotation impurit

Info

Publication number: GB1570345A
Application number: GB14877/77A
Authority: GB
Original assignee: Swemac SA
Current assignee: Swemac SA
Priority date: 1976-04-12
Filing date: 1977-04-07
Publication date: 1980-07-02
Also published as: ATA247877A; NL8601174A; SE452121B; FR2544225B1; FI68093B; NL7703627A; FI68093C; FR2544225A1; CA1096515A; ES458029A1; DE2712947A1; FI770993A; FR2347984B1; JPS56161849A; JPS6057903B2; IT1218334B; IT8348476A0; NL180076B; DE2712947C2; DE2760246C2

Abstract

A floatation tower for paper recovery has a set of cells one above the other, holding the fibrous paper paste, and into which water and compressed air are injected so that the impurities mix. with the bubbles to form a foam which can be removed separately. Water is admitted as a thin layer at the same time as the air to mix. with the liquid paste as it is circulated in a thin layer in a mixing chamber, passing it through and along the flow, controlling the thickness of the layer. Air is then admitted to skin off the foam and drive it into a collector vessel below, for recycling. By a similar process for each cell, ink, kaolin and fibres can be removed separately. The mixing chamber is conical, with a tangential lop entry.

Description

(54) PROCESS AND APPARATUS FOR ELIMINATING BY FLOTATION IMPURITIES IN THE FORM OF SOLID PARTICLES CONTAINED IN A LIQUID (71) We, SWEMAC SOCIETE ANONYME, a Belgiun Societe Anonyme of rue Vilain XIIII, 55 - 1050 Bruxelles, Belgium, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it Is to be performed, to be particularly described in and by the following statement:: The present invention relates to a process and apparatus 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 in which process a stream of air is injected into a flow of the liquid in question in 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.

A flotation process of the kind described above is 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 the 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 flotation, 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 different bubble dimensions for the flotation of the inks, for the flotation of fillers such as kaolin, or for the flotation of fibres, and thus to proceed to their selective separation.

The aim of the present invention is to provide a process providing for the adjustment of the bubble dimensions independently of the volume of air mixed with the fluid which is to be purified.

Accordingly the invention provides 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 with the impurities to the surface in such a manner as to produce a foam which is charged with these impurities and which is discharged from the surface of the liquid, 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 simultaneously 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.

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 layer in a narrow chamber and its encounter with divided air injected transversely on to the fine layer of liquid charged with suspended fibres give rise to an effect of rubbing and cleaning these fibres.

Preferably the thickness of the layer of liquid in the chamber and consequently the speed of passage of the liquid in the chamber are adjustable.

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 adjusting them it is possible to obtain differences of whiteness ranging from 57Q to 66.50 Scan.

These adjustments make it possible to achieve effective purification with minimum rejection.

Conveniently, the length of the path of admission of air into the liquid to be treated is adjustable. It is thus possible for the amount of air to be accurately proportioned in relation to the layer of liquid passing through the mixing chamber.

Preferably, 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.

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 by adjustment of the airflow to the mixing chamber 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 flota tioncell.

The present invention also provides an apparatus 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 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 parallel walls forming a flow passage for the liquid through the chamber, at least one of the walls having pores therethrough, aerating means 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 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.

The mixing chamber is bounded by two parallel walls spaced at a distance from one another, so as to allow the passage of liquid to be purified.

The mixing chamber is also be provided with a means for adjusting the distance between the parallel walls.

Embodiments of a flotation apparatus, which is particularly suitable for the deinking of pulp prepared from old papers, will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a diagrammatical view in elevation of a multi-storey flotation apparatus; Figure 2 is a horizontal section taken along the line 11-11 in Figure 1; Figure 3 shows on a larger scale a view in longitudinal vertical section of a mixing chamber; Figure 4 is a sectional view taken along the line IV-IV in Figure 3, Figure 5 shows on a larger scale and in vertical section a modified form of construction of a mixing chamber; Figure 6 is a sectional view taken along the line VI-VI in Figure 5; and Figure 7 is a diagram showing two whiteness curves.

In these drawings the same reference numerals designate identical elements.

The flotation apparatus comprises four flotation cells 1, 2, 3 and 4, which are of annular shape and are axially superimposed so as to constitute four stages of a tower having a plurality of floors 5. Obviously this tower could have a number of stages different from four.

Defibred, chemically pretreated paper pulp in the form of a fibrous suspension containing a foaming agent is delivered by means of a pump 6 through a pipe 7 to the inlet of the cell 1, first passing through a mixing chamber 8. This chamber has a narrow 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 suspension 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 annular cell. The bubbles created in the suspension fix themselves to the impurities and rise together with the impurities to the surface in the form of foams. The latter are entrained out of the cell through the outlet 18 provided in the pipe 12, partially through the rotational movement of the suspension and partially by a current of air provided above them. A vertical plate 50 disposed above the level of the pulp guide the foams to the outlet. The forms 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 pipe 14 connected to a suction device 15 disposed above an axial pipe 16 comprising a frustoconical part and a lower part having a coneshaped upper end located below the frustoconical part 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 discharge 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 2, forming 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 from a cyclone, thus imparting a centrifugal velocity to the foam, permitting the separation of the air which is drawn into this pipe by the action of the suction device 15, whence it is returned to the pipe 14 for reuse for the discharge of foams.

The pulp partially cleaned in the cell 1 leaves the latter through a pipe 19 provided with a pump 20 enabling 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 utilized in the first cell, and so on in the other cells. The latter are fed successively with the pulp or water containing fibres of an increasing degree of purity, while the cleaned liquid collected in these other cells is 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 recycled or discharged at each stage for treatment in a separate device, depending on the type of product for which the pulp is intended.

The apparatus also comprises a cover 40 closing the cell of the last stage, an inspection window 41 permitting rapid discharge for cleaning purposes; an emptying valve 42 for each cell; a flap 46 inthe foam transporting air pipe; a floor 43 in 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 for the discharge of the accepted pulp in the cell 2 treating the mostly highly purified pulp.

Figures 3, 4 and 5, 6 illustrate respectively two forms 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 parallel plane walls 21, 22 mounted in the 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 adjacent wall of the parallelepipedic body permitting the adjustment of the distance between the parallel walls by means of a wheel 25, in order to determine the thickness of 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 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 provided in the fritted plate so as to regulate the air flow admission surface.

In the modified 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 nozzle 31 leading tangentially into the chamber through the wall 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). The inner cone 30 is adapted to slide axially in the outer cone 29 so as to adjust the space between these walls and consequently the thickness of the 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 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 admiting 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 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.

Although the invention which has been described relates to an apparatus for the de-inking of paper pulp, it could be used for other applications, for example for the purification of ores or for the purification of white water by flotation of the fibres.

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 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 4.23 metres per second, while the curve 49 shows a pulp whiteness for a pulp speed of 6.35 metres per second.

By adjustment of the air-pulp mixture and of the speed of the pulp in the mixing chamber it is possible to obtain differences of whiteness ranging from 57" to 66.50 Scan. The tests were carried out with old papers of the "magazine" type, containing about 50% of mechanical pulp having an original whiteness of the unprinted edges of 66.5 Scan.

By applying the process of the invention the whiteness of the printed parts containing about 2% of ink can be increased from 46 to 66.50 Scan, with a retention of 20 minutes. After 2.5 minutes retantion the whiteness has already passed from 46" Scaid'to to 64.5 Scan. The best present con- ventional 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.

WHAT WE CLAIM IS: 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 time the air is admitted simultaneously into this liquid layer transversely of the path 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 form 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 any of claims 1 to 3, in the case of flotation with a plurality of flotation cells, wherein a mixture of liquid and air coming from a mixing chamber 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 of 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 containing at least one foaming agent 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

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. 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). The inner cone 30 is adapted to slide axially in the outer cone 29 so as to adjust the space between these walls and consequently the thickness of the 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 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 admiting 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 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. Although the invention which has been described relates to an apparatus for the de-inking of paper pulp, it could be used for other applications, for example for the purification of ores or for the purification of white water by flotation of the fibres. 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 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 4.23 metres per second, while the curve 49 shows a pulp whiteness for a pulp speed of 6.35 metres per second. By adjustment of the air-pulp mixture and of the speed of the pulp in the mixing chamber it is possible to obtain differences of whiteness ranging from 57" to 66.50 Scan. The tests were carried out with old papers of the "magazine" type, containing about 50% of mechanical pulp having an original whiteness of the unprinted edges of 66.5 Scan. By applying the process of the invention the whiteness of the printed parts containing about 2% of ink can be increased from 46 to 66.50 Scan, with a retention of 20 minutes. After 2.5 minutes retantion the whiteness has already passed from 46" Scaid'to to 64.5 Scan. The best present con- ventional 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. WHAT WE CLAIM IS:

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 time the air is admitted simultaneously into this liquid layer transversely of the path 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.

to form bubbles therein, wherein the mixing chamber comprises opposed parallel walls forming a flow passage for the liquid through the chamber, at least one of the walls having pores therethrough, aerating means 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. An apparatus according to claim 7, wherein the walls are conically shaped and coaxial, and the adjustment means moves the walls axially relative to one another.

9. An apparatus according to claim 8, wherein the inlet means is arranged to provide a tangential flow of the liquid into the passage.

10. An apparatus according to either of claims 7 and 8, 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.

11. An apparatus according to either of claims 7 and 8, wherein it includes means for controlling the flow rates of liquid to the inlet means and of air to the aeration means.

12. An apparatus according to either of claims 7 and 8, wherein it includes means adjustable to close a plurality of the pores.

13. An apparatus for elimination by flotation of solid particle impurities contained in a liquid which comprises at least one enclosed 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 parallel walls forming a flow passage for the liquid through the chamber, at least one of the walls having pores therethrough, aerating means providing a stream of air through the pores to form bubbles in the liquid flowing through the passage which attach to and float impurities to form a surface foam on the liquid in the cell, adjusting 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 liquid through the passage, thereby inversely changing the size of the bubbles, outlets for separately discharging a surface foam on the liquid in the cell and cleaned liquid from the cell and foam ejection means for directing a stream of air into the surface foam, to blow the foam toward and out of its outlet, the form ejection means further including suction means and duct means for recyling the air of the airstream.

14. An apparatus according to claim 13, 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.

15. An apparatus according to claim 14, 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.

16. An apparatus according to claim 15, 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.

17. An apparatus according to claim 16, 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.

18. An apparatus according to claim 17, 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.

19. An apparatus according to claim 18, 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 outlet of cleaned liquid of each cell except one, to the inlet of another cell to provide series treatments of the liquid in the plurality of cells.

20. An apparatus according to claim 17, wherein it is provided with an axial pipe for the recycling of the foam discharge air, which pipe comminicates with all the cells.

21. An apparatus according to any of claims 17 to 20 wherein the axial pipe is in two parts, the part recycling form discharge air from the bottom cell extends into the adjacent cell forming a cone in the upward direction and disposed facing a wider end of other part for recycling the foam discharge air of the adjacent cell and of the other cells.

22. A process for the elimination by flotation of impurities substantially as herein described with reference to the accompanying drawings.

23. An apparatus for the elimination by flotation of impurities substantially as herein described with reference to the accompanying drawings.