Nowadays it is common practice for paper to be collected separately after use, in order to be recycled into new paper. The collected waste paper comprises newspapers, magazines, cardboard boxes and so forth and is usually printed with various types of ink. In addition, depending on the nature of the waste paper, it will comprise several other contaminants, such as filler compounds. It is necessary that the various contaminants present in the waste paper are removed if it is desired to produce new (i. e. recycled) paper which is sufficiently white in color. Depending on the intended application of the recycled paper, the extent in which the contaminants have to removed will vary. For applications such as stationery or paper for glossy magazines a very high degree of whiteness is required. For other applications, such as newsprint paper, paper having a more or less gray shade may be acceptable.

In the past several different methods for removing contaminants from waste paper have been proposed. An overview of these methods may be found in"Technology of Paper Recycling", Ed. R. W. J. McKinney, Chapman & Hall, 1995. A method that has received especially much attention is flotation deinking.

In 1903 it was found that air bubbles would pick up chemically coated mineral particles in a weak acid solution.

This discovery lead to the development of flotation in mineral ore dressing. In the early 1930's it was proposed that these flotation techniques could be applied to deinking of waste paper. Flotation was reported to achieve better brightness results than washing and it was suggested that the best approach would be a combination of washing and flotation. Several designs of equipment have been proposed in

the literature, of which the first were derived from those used in mineral ore flotation.

The concept of flotation deinking is based on the difference in hydrophobic nature of the paper fibers and the ink. With the exception of water-based (including flexographic) inks, ink particles are more hydrophobic than paper fibers. In some cases, chemicals are added to increase the difference in hydrophobic nature. Further, it has been found that the deinking of magazines and newspapers combined leads to better results than the deinking of newspapers alone. Due to the chemical complexity of the process, an explanation for this phenomenon has not yet been found.

In order for the flotation process to be successful, a number of interactions must occur. First of all, the ink particles must be sufficiently free to have the opportunity to collide with an air bubble. The ink particles should thus not be attached or bound to fiber surfaces, or trapped in fibrillar areas. Further, the collision between an ink particle and an air bubble must provide sufficient energy for a complex to be formed, which complex must rise to the surface, along with the other air bubbles, and be removed.

According to McKinney, it is necessary that the air bubbles used have a minimal diameter of approximately half a centimeter. Smaller bubbles, as are used in dissolved air flotation, would reduce the chance of collision with an ink particle too much and tend to increase the loss of paper fibers. Furthermore, a high turbulence because of for instance increased air injection should, according to McKinney, be avoided, as this could lead to the breaking up of the ink-bubble complexes and might prevent the complexes from rising to the surface.

The conventional flotation deinking process is described, inter alia, in the European patent applications 0 798 416,0 537 416 and 0 618 012. In the latter document, a deinking cyclone is used, wherein the separation between contaminants and paper fibers is based on the centrifugal

action of the device. EP-A-0 798 416 discloses a method wherein contaminants and paper fibers are separated from each other in a flotation method, while water is being sprayed on top of the waste paper suspension. In EP-A-0 537 416, a tertiary flotation treatment of waste paper is described. In this treatment, paper fibers are recovered, which were removed in the flotation from the waste paper suspension along with contaminants.

In practice, it has been observed that in a conventional flotation process a rather large amount (up to 5%) of the desired paper fibers is lost. Also, it has been found that in flotation methods a rather large amount of ink remains in the pulp in a flotation process. In other words, the selectivity of the deinking process is not very high.

Hence, it is desired that an improved process for the rernoval of contaminants of waste paper having a higher selectivity and a reduced loss of paper fiber is provided.

Surprisingly, it has now been found that both the efficiency and selectivity of a process for removing contaminants, such as ink, from waste paper can be improved by making use of a foam fractionation technique. Based on said technique, the invention relates to a process for removing contaminants from waste paper comprising a) leading an inert gas through an aqueous pulp in a vessel, which pulp comprises waste paper fibers and contaminants, to produce a foam comprising at least part of the contaminants, which foam forms a layer on top of the pulp, and b) removing foam from the vessel.

Compared to the conventional flotation techniques, in a process according to the invention a reduction in fiber loss of up to 50% is achieved. Further, the amount of contaminants that is removed in a process according to the invention may be significantly improved.

In the foam layer that is formed in a process according to the invention, two gradients have been observed.

In the most upward regions of the foam layer, lower concentrations of paper fibers occur in the foam when compared to the downward regions of the foam layer. For the concentration of contaminants in the foam layer, a gradient in the opposite direction has been observed. Thus, the foam layer establishes selectivity in a process of removing contaminants from the waste paper.

The waste paper that is purified in a process according to the invention may be any form of paper that has been used for any sort of purpose. Suitable stocks include newspapers, magazines, cardboard boxes and so forth. These stocks may be processed separately or combined. Preferably, a mixture of different sorts of waste paper (for instance both newspapers and magazines) is purified, which is believed to lead to a more efficient removal of contaminants. Moreover, a labor and/or cost intensive sorting step may thus be avoided.

The waste paper is processed in the form of an aqueous pulp. In order to form this pulp, the waste paper is mixed with a suitable amount of water, and the obtained mixture is stirred, minced and/or chopped, so that the paper product mainly falls apart into paper fibers. The amount of water added will usually be chosen such that a pulp is obtained which comprises of from 0.5 to 2.5 weight percent of paper fibers, based on the pulp. It is preferred that the pulp comprises more than 1 weight percent of paper fibers. In a highly preferred embodiment, the pulp comprises more than 1.5 weight percent of waste paper fibers. Surprisingly, it has been found that in accordance with the invention it is possible to process pulps having a relatively high paper fiber consistency (or concentration). As a consequence, the capacity of a process according to the invention is very high.

The aqueous pulp will generally comprise (paper) fibers, filler compounds and ink. When the pulp is formed from waste paper directly after having been collected after its use, the pulp will often also comprise other materials,

which may have been part of a paper product, such as metals (from staples and the like) or adhesives (from labels, book bindings and the like), but also materials which accidentally or erroneously have been discarded in the same effluent stream as the waste paper, such as glass or plastics. These materials may be removed from the pulp in a conventional manner, e. g. by use of a cyclone and/or a sieve, before a process according to the invention is carried out.

According to the invention, contaminants are removed from the aqueous pulp by using a foam fractionation technique. To this end, an inert gas is lead through the pulp to produce a foam layer on top of the pulp. As has been described hereinabove, in conventional flotation processes air is bubbled through an aqueous pulp of waste paper. An important difference between the present process and these flotation processes, is that a foam layer of considerable height is produced. Thus, it is preferred that the ratio of the height of the foam layer to the height of the pulp layer is at least three, preferably at least five, more preferably at least 10. In accordance with the invention it has been found that a highly selective removal of contaminants is achieved by performing the process such that a foam layer of at least 30 cm high is produced. Even better results are achieved when a foam layer of even greater height is produced. Hence, it is preferred that a foam layer of at least 50 cm, more preferred at least 100 cm, is produced.

Another difference between the conventional flotation processes and a process according to the invention is that the size of the air bubbles is much less relevant. In order to achieve sufficient interaction between an air bubble and an ink particle in a flotation process it is necessary to optimize the size of the air bubbles used. Further, the residence time of the paper pulp in a process according to the invention may be shorter than that in a flotation process. Typical residence times in a process according to the invention are between 1 and 4 minutes, which is in the

order of a factor 2-3 less than in conventional flotation processes.

The inert gas may be any gas which does not chemically react with the materials present in the pulp and which is capable of physically interacting with the contaminants to be removed. Suitable gases include air, and nitrogen gas. For economic reasons, air will generally be used as the inert gas. The amount of gas used will be adjusted to generate the desired amount of foam in a manner which is within the normal skill of the skilled person.

Generally, the amount of added gas will be at least 1 liter per liter of the added pulp. In other words, the ratio of the gas flow with respect to the pulp flow will preferably be at least 100 vol. %, preferably at least 200 vol. %. For practical reasons, said ratio will usually not exceed 500 vol. %.

A process according to the invention can suitably be carried out at a temperature of from 25 to 45°C. Preferably, the temperature is chosen around 35°C. It has been found that temperatures within the specified ranges have a beneficial on the formation of a foam.

In a preferred embodiment, water is sprayed on top of the foam layer. The result of the spraying of the water is that an even smaller amount of paper fibers will be removed together with the foam. In other words, the selectivity of the process is enhanced by the spraying of the water.

Preferably, the spraying is performed in such a manner that the amount of water sprayed is evenly distributed over the upper surface of the foam layer. The amount of water sprayed is adjusted so that the foam does not collapse under the force of the falling water, but that the production of foam is optimally enhanced. Preferably, the ratio of the flow of the spraying water with respect to the pulp flow in the process is below 5 vol. %.

In another preferred embodiment, the aqueous pulp is kept in turbulent conditions. In contrast to what is described by Mckinney, it has now been found that it is

advantageous to effect a high turbulence in the pulp. Due to the turbulence, a pulp having a higher fiber consistency (concentration) can be processed. The desired turbulence may be created in any known manner. It is for instance possible to stir or shake the pulp, or to increase the flow (velocity) of the inlet gas stream. It is also possible to perform the present process in a vessel comprising protuberances in one or more side-walls, which have the effect that the inert gas that is lead through the pulp creates a turbulence. This latter embodiment will be further elucidated hereinbelow. It has further been found that a high turbulence is beneficial to the formation of foam. Also, homogenization of the pulp will prevent the flocculation of the paper fibers in the pulp. Flocculation may result in encapsulation of contaminants in fiber aggregates, because of which a certain amount of contaminants will not be removed in the process.

A process according to the invention can suitably be carried out in a vessel comprising an upward tapering, for example frustoconical first compartment, a second compartment extending from the open top side upwardly, like a neck. The second compartment has an open top side and a vertical length which is relatively large compared to the height of the first compartment; whereas the width of the second compartment is small relative to the maximal width of the first compartment.

The first compartment is provided with an inlet and outlet for the aqueous pulp. Furthermore, air inlet openings, for example air jets are provided in the first compartment, preferably at least in or near the bottom thereof, for providing streams of air bubbles through the first compartment. Means are provided for creating a high turbulence in the aqueous pulp in the first compartment, for example stirring means, whereas the high turbulence can also be created by means of the air jets. Due to the layout and inter alia the high turbulence in the aqueous pulp and the air inlet foam is being created in at least the second compartment and, preferably, near the tapering top side of

the first compartment. Due to the relatively large height of the second compartment and the narrow cross section thereof a foam layer having a large height is being created, providing for sufficient fractionation within the foam. Over the open top end of the second compartment spraying means are provided for spraying water on top of the foam layer, for even further enhancing fractionation within the foam. Furthermore, means are provided for skimming at least the top fraction of the foam layer, which means can be of any suitable form, for example blowing means, scraping means or an overflow and suction means or the like. Such variations will be directly clear to a person skilled in the art.

From the European patent application 0 146 235, a device is known for mineral separation by using foam fractionation. The device comprises two frustoconical compartments, extending upwardly like a neck. The vertical lengths on top of these frustoconical compartments are relatively short in comparison with the vessel compartment beneath the two frustoconical compartments.

As has been explained above, in a process according to he invention, contaminants present in an aqueous pulp comprising waste paper fibers are removed from said pulp by leading an inert gas through the pulp to form a foam layer.

The paper fibers which are intended to be recycled into new paper remain in the pulp and the contaminants rise up with the inert gas into the foam. However, there may be a very small fraction of the paper fibers that rises up with the foam, and is removed along with it. Preferably, these fibers removed with the foam are recovered. Thus, in a preferred embodiment at least the foam fraction skimmed from the second compartment is introduced into a further foam fractionating device according to the present invention, for removing at least part and preferably almost all of the fibers left in the foam.

After the contaminants are removed from the pulp by removal of foam comprising said contaminants to a sufficient

degree, the pumping of gas through the pulp may be terminated. However, preferably the process is carried out continuously, which means that there is a constant stream of pulp into the first compartment of the vessel, and a constant stream of pulp from which contaminants have been removed out of the vessel. The latter pulp will subsequently be further processed in process steps conventional in the field of paper recycling into new paper products.

During these subsequent, conventional process steps, several aqueous effluent streams are obtained. Examples of these effluent streams are wash water and thickener water. It is normal procedure to isolate the ash, comprising among others filler compounds, and other solid materials (usually referred to as Total Suspended Solids, TSS) present in these effluent streams.

For the isolation of the ash from the water it is common practice to make use of a technique called Dissolved Air Flotation (DAF). This technique is somewhat similar to the above described flotation technique, in that air is lead through an aqueous system. However, in DAF the size of the air. bubbles lead through the system is extremely small. Also, generally certain chemicals, mostly poly-electrolytes, are added to the system in order to facilitate the removal of the solid materials from the water. When DAF is used to isolate solids from the above mentioned effluent streams, these solids will adhere to the poly-electrolytes and flocculate.

The formed aggregates will rise to the surface of the system, where they will form a sludge. This sludge is subsequently removed from the system. For a more detailed discussion of DAF reference is made to B. Carre,"Dissolved air flotation of process water in flotation deinking", Centre Technique du Papier,'The third advanced training course on deinking technology', March 1997, Grenoble, France.

As a result of the DAF, a water stream is obtained which is sufficiently pure to be re-used somewhere in the paper recycling process, for instance for dilution purposes,

or to be discarded into the environment without giving rise to unacceptable pollution problems. The obtained ash may be dispensed with in the usual manner.

A great disadvantage of the DAF technique is that it does not show any selectivity. Simply all solid material present in the aqueous system will be removed. As many of the above effluent streams, which are conventionally subjected to a DAF, comprise a considerable amount of paper fibers, it would be desirable to be able to recover these fibers.

Surprisingly, it has now been found that the above described process of the invention for the removal of ink from waste paper can also be used to selectively remove contaminants, such as filler compounds, from an effluent stream of a process for recycling paper. In this process, the filler compounds and so forth are removed with the foam and the paper fibers present in the effluent stream remain in the aqueous pulp. This application of the present invention to effluent streams of a paper recycling process significantly increases the efficiency of said recycling process.

The pulp from which the filler compounds and so forth have been removed, obtained in this recovery process is preferably re-used in the recycling paper process. The foam comprising the contaminants may be discarded.

Of course, the invention also relates to the waste paper fibers that are purified, and to articles of recycled paper manufactured therefrom, in a process as outlined hereinabove.

Figure 1 shows schematically a device according to the present invention; and Figure 2 shows schematically a paper recycling unit according to the present invention.

Throughout this description corresponding parts will have corresponding reference signs.

Figure 1 shows schematically, in cross section a fractionating device 1, comprising a first compartment 2 and a second compartment 3. The first compartment 2 has a

substantially flat bottom 4 and two sidewalls 5, tapering upward and enclosing the upward tapering first compartment 2.

The first compartment 2 can for example be frustoconical but is preferably elongated in the direction perpendicular to the plane of the drawing, having flat or also tapering endwalls (not shown). In Figure 1 on the left-hand side the first compartment 2 is provided with an inlet 6 for an aqueous pulp, which can be introduced under pressure by means of first pumping means 7. On the opposite side near the bottom 4 an outlet 8 is provided for the aqueous pulp, after treatment within the first compartment 2. Second pumping means 9 can be provided near the outlet 8. At top 10, the first compartment 2 is connected to the second compartment 3. The second compartment 3 has a width W considerably smaller than the maximum width D of the bottom 4 of the first compartment. The height H of the second compartment 3 is considerably larger than the height T of the first compartment. The volume of the first compartment can be comparable to or smaller than the volume of the second compartment 3. The top end 11 of the second compartment 3 is open. Spraying means 12 are positioned above the open top end 11. A casing 13 is provided over the top end 11 and the spraying means 12, from which casing 13 foam, developed in the second compartment 3, can be extracted by means of third pumping means 14, connected to the casing 13 at outlet 15.

In the bottom 4 of the first compartment 2 first air inlet means 16 are provided, preferably in the form of air nozzles, for providing streams of air bubbles through the aqueous pulp within the first compartment 2. Second air inlet means 16A can be provided in for example the sidewalls 5.

With the air inlet means 16,16A a relatively large amount of air is introduced into the aqueous pulp. The volume air introduced into the first compartment per hour can for example be equal to or even larger than the volume of aqueous pulp being treated within the device during the same time.

The layout of the fractionating device, especially of the first compartment 2 is such that a high turbulence is created within the aqueous pulp in the first compartment. To this end protuberances 17 can be positioned on the inside of for example the sidewalls 5. Furthermore, stirring means or the like (not shown) can be provided within the first compartment 2. Due to this layout a turbulent stream of aqueous pulp is created within the first compartment 2 between the inlet 6 and the outlet 8. Fourth pumping means 18 are provided for directing air or any other suitable inert gas to the various air inlet means 16,16A, wherein the layout of the air inlet 16,16A can be such as to increase the turbulence within the aqueous pulp.

Figure 1A shows a preferred layout of an inlet, which may be used as the inlet 6 for the pulp or the air inlet 16.

The inlet 6 or 16 is provided in the bottom 4 or a side wall 5, having an outlet 19 for pulp or air within the first compartment 2 and a first inlet 20 for pulp or air, connected to pumping means 7 or the fourth pumping means 18, respectively. A second inlet 21 is connected to the inlet means 6,16 between the first inlet 20 and the outlet 19.

When during use pulp or air is pumped through the outlet 19 by means of pumping means 7 or the fourth pumping means 18, external water or pulp, in case the inlet means 16 are used as the inlet for pulp 6, or external air, in case the inlet means 16 are used as the inlet for air, is sucked into the inlet means 6,16 and forced into the first compartment 2.

These inlet means 6,16 have the advantage that with relatively small pumping means 7 or fourth pumping means 18, respectively a relatively large amount of pulp or air can be introduced into the first compartment.

Figure 2 shows schematically a paper pulp treatment plant 22 comprising a number of fractionating devices 1 according to the present invention, and for example shown in Figure 1. The plant 22 comprises in line, leading to the inlet 6 of a first fractionating device 1A subsequently a

pulper 23, a cyclone 24 and a sieve 25. In a continuous stream an aqueous pulp is produced in the pulper 23, which pulp is fed in the cyclone 24 for distracting any unwanted material, such as metal staples, paper clips and the like, after which the aqueous pulp is fed through the sieve 25 for distracting for example labels, bags and the like. The aqueous pulp thus obtained is fed into the first compartment, creating a high turbulence therein. Air is fed through the aqueous pulp, the air bubbles extracting the ink therefrom. A foam layer with a large height H is created within the second compartment, providing for a optimal fractionating there within. The foam is skimmed from the top end 11 of the second compartment 3 and, by means of the third pumping means 14 provided to the inlet 6 of an optional second fractionating device lA, comparable to the first fractionating device 1.

The foam of the second fractionating device 1A is discarded, for example burned, whereas the aqueous pulp, extracted from the outlet 8 is fed into the inlet 6 of the first fractionating device 1. Thus any fibers left in the foam layer of the first fractionating device 1 will be recycled into the first compartment of the first fractionating device 1 through the second fractionating device 1A.

The aqueous pulp distracted from the first compartment 2 of the first fractionating device 1 through the outlet 8 is fed into a second cyclone 26, distracting unwanted particles from the aqueous pulp, such as sand, after which the aqueous pulp is fed into a washer or thickener device 27, in which a large amount of water is pressed from the fibers. The distracted water is fed into a third fractionating device 1B for a further fractionating of the aqueous pulp. The foam is discarded from the third fractionating device 1B, whereas the effluent thereof is recycled into the stream of aqueous pulp to be fed into the first fractionating device 1.

The paper fibers resulting from the washer or thickener device 27 are fed into a paper making device 28,

for further treatment. Thereby, once again water is pressed from the fibers, which water is fed into a fourth fractionating device 1C, for further treatment. The foam of the fourth fractionating device 1C is also discharged, whereas the effluent thereof is recycled into the aqueous pulp stream.

A large number of alternative embodiments of a device according to the present invention is possible within the frame of the invention. For example, the casing 13 can be replaced by other means for skimming the foam layer, such as means for blowing away part of the foam layer to distracting means. Especially when the first 2 and second compartment 3 are in an elongated form the foam distracting means can advantageously be provided as scraping means, movable along the top side 11 of the second compartment 3. With these scraping means the top layer of the foam layer can be scraped or lifted from the second compartment 3 and be pushed or carried or the like to the third pumping means 14.

Furthermore, the first to end second compartment 3 can have different shapes and dimensions, depending inter alia on the desired turbulence in the aqueous pulp, the concentration of said pulp and the type and amount of pulp to be processed.

By way of example a number of dimensions of a first fractionating device 1 according to the present invention are given, which should in no way be regarded as limiting the scope. The height T of the first compartment can be for example up to 75 centimeters, the height H of the second compartment being more than 30 centimeters, preferably between 1 and 2 meters. The width W of the second compartment is for example, 25 centimeters, the width B of the bottom 4 of the first compartment 75 centimeters. As will be clear to the skilled person, these dimensions can suitably be adapted to the desired capacity of the device. However, the above dimensions provide some guidance as to a suitable relation between the various dimensions of the device.

The invention will now be further elucidated by the following, non-restrictive examples.

Example I In a cylindrical vessel, having a diameter of 30 cm, a conventional flotation process, comprising a primary and a secondary flotation, of a pulp from wood free waste paper (heavily bleached sulfate board) comprising 1.18% of paper fibers and having an ash content of 17.4% was carried out.

The inlet stream of air was 15 1/min. The residence time was 10 min., and the air to pulp ratio was 30%.

The ash content of the reject was 55%, which corresponds to a loss of ash from the pulp of 59%. The loss of fiber amounted to 15%.

Example II In a cylindrical vessel, having a diameter of 30 cm, a process according to the invention was carried out using various pulp flows and an air inlet stream of 18 1/min.

The process parameters are shown in Table I.

Table I height of air/pulp residence height of consistency (%) pulp layer ratio time foam layer (cm) (%) (min) (cm) in out re- ject 75 48 1. 30 95 1. 18 1. 15 0.23 83 108 3. 26 87 1. 04 0. 89 0.27 72 141 3. 72 98 1. 15 0. 98 0.21

The results are shown in Table II Table II loss of ash (%) loss of fibers (%) ash content (%) in out reject 15.7 1. 8 17. 4 15. 3 64.4 39.2 3. 7 17. 6 11. 9 69.4 38.2 3. 7 16. 4 11. 2 66.8 Example III A procedure in accordance with the invention was carried out in a frustoconically shaped vessel, of which the layout correspondeded to the vessel as shown in Figure 1. The height of the first compartment was 50 centimeters. The second compartment was 100 cm high and 25 cm wide.

The used pulp was from wood free waste paper (heavily bleached sulfate board) which comprised 1.18% of paper fibers and an ash content of 17.4%. The inlet air stream was 18 1/min., the air to pulp ratio was 30%, and the residence time was 1.30 min.

During the process, a foam layer having a height of more than 1 meter was observed. From the whiteness of the paper fibers obtained in the process, it was clear that the amount of ink removed from the pulp was higher than in the experiment of which the process parameters and the results are shown in the first entries of Table I and II.