METHOD FOR DEINKING AND OTHER CONTAMINENT REMOVAL FROM WASTEPAPER BACKGROUND OF THE INVENTION This invention relates to the deinking and removal of stickies and other hydrophobic contaminants from repulped waste paper. A sticky can be any tacky impurity in a paper stock suspension, and as such is commonly found as a result of the repulping of old paper material. Generally, a sticky is in the form of a discrete particle or groups of such particles clumped together and usually of a size that is visible to the naked eye as well as in any hand sheet made from the pulp containing stickies. Particularly, while stickies may be formed at times by wood resin in new pulp, when old paper is repulped, the stickies are generally very small pieces of plastic materials and, like ink and carbon particles, they are generally of a hydrophobic nature. The fact that they are inherently hydrophobic enhances the ability to extract such contaminants by a froth flotation processes. Such particles that are highly hydrophobic do not generally require the addition of conditioning agents, surfactants, and pH adjusting agents or these agents, may be reduced in quantity and applied only as necessary.

In deinking of waste papers, the hydrophobic contaminants are attached to the paper and must be debonded from it to permit separation and removal from the pulp slurry. Conventionally, the debonding occurs in the pulper, in the presence of chemical additives such as surfactants, detergents, deinking chemicals, and sometimes caustic. These chemicals act similarly to a laundry process. Once debonded and suspended, the particles are conventionally separated in a de-ink flotation step, with the aid of air bubbles. The particles attach to the bubbles and are carried to the slurry surface, where they form a foam that can be removed. For good removal efficiency, additional de-ink chemicals may be added ("displectors", saponified fatty acid, or generic nonionic dispersers/collectors, for example).

Unfortunately, flotation efficiency varies greatly. Sometimes, large particles flotate poorly. Stickies are hydrophobic and should flotate readily, but do not. Changes in the chemical environment such as dissolved solids in the water,

water hardness, and changes in raw material, etc., can dramatically reduce flotation efficiency.

Also, it is not clearly understood why deinking chemicals are required to activate already hydrophobic stickies, so that they can stick to air bubbles. Such attachment should be possible without chemicals. Actually, certain deinking chemicals have hydrophobic and hydrophillic ends. If the hydrophobic ends attach themselves to the hydrophobic particle, they actually render it hydrophilic, which is not desirable. Accordingly, deinking flotation chemistry is a poorly understood science and practice.

An explanation for the shortfall in removal efficiency may be that the hydrophobic particles (ink, stickies, etc.), which should float easily, lose their hydrophobicity in the wet process before they arrive at the flotation cell. This may be due to --the chemicals which are added in the pulper, or afterwards, prior to flotation, --the other dissolved solids which are in the repulping water, the water itself, --the chemistry of the particle surface which is differently affected by the various chemicals in this environment, and/or --the porosity of the particle and/or its inherent ability to absorb water and bulk.

The mechanism of hydrophobic attraction between a particle and an air bubble has been observed. A particle slowly approaches an air bubble. When a critical proximity is reached, the particle support bends to meet the bubble surface, and the bubble surface itself deforms toward the particle. This occurs quickly and appears like a jump. The observed process is not unlike a ferrous particle approaching a magnet.

When one tries to separate the particle from the bubble by moving the particle away again, considerable attraction forces must be present, because the bubble now bulges outwardly, to hold on to the particle. Likewise, the particle holder deflects strongly toward the bubble, in response to the holding force.

When such an experiment is repeated, even after a few minutes, or a few hours of resilience or soaking time, the effect is less pronounced and after yet further time the attraction force is absent. The particle can virtually be pressed into the bubble surface, to deflect it inwardly, but there is no attachment. This is apparent when the bubble and particles are slowly separated. It can be seen that the bubble resumes its original shape and there is no attractive force between particle and bubble.

Accordingly, it must be assumed that hydrophobic parties in a water suspension can lose their hydrophobicity, over time. The invention presented here is a practical utilization of these observations.

SUMMARY OF THE INVENTION The invention includes methods of removal of hydrophobic ink and stickies, or other hydrophobic particles from a suspension of repulped old paper product, whereby the hydrophobic nature of the particle surface is preserved, at least until the flotation deinking step. This objective is achieved by applying one or more of the following principles: --Minimizing addition of chemicals which could destroy the hydrophobicity.

--Minimizing exposure to water both as to amount and time.

--Minimizing the time of contact with water, from the beginning of the disintegration to the flotation deinking step.

All three of the above principles are contrary to conventional deinking technology, because water and chemicals are believed to be needed for the debonding of the particles from the paper fibers. Water is needed for weakening the paper, so that it can be readily defibered and disintegrated. Residence time of contact is provided in the pulper and in equalization chests. Sometimes, the industry practice even calls for soaking towers where the paper slurry is held for many hours. After such time, a gentle action can defiber the now weakened paper, while preserving the physical integrity of the contaminants. No thought is given to the hydrophobic nature of the surfaces of the contaminant particles.

Other factors in repulping and cleaning of wastepaper cannot be ignore. Some water is required to weaken the paper and facilitate disintegration by repulping, and the separation of the contaminants from paper fibers. Also, some chemicals which increase wetability or hydrophyllicity may also be required for the detachment of the contaminants from the paper fiber. Surfactants may be required to produce a sufficiently stable foam in certain flotation cells to permit the removal of the contaminants, together with the foam. In the spirit of the invention, it is not necessary to use the absolute minimum conditions, i. e., time and water against loss of hydrophobicity. Rather, it is the concept to provide optimum conditions for the removal of hydrophobic contaminants. These conditions are far different from the conditions of current practice, as illustrated herein.

In a further aspect of the invention, deinking is accomplished in hydrocyclone cleaners. Flotation deinking in hydrocyclone cleaners has been proposed and tried on an exploratory basis with air being added at the pump suction feeding the reverse cleaner. No records of such trials are known to exist as these apparently were not successful.

As an aspect of this invention, a dissolved air step may be added prior to applying material to a hydrocyclone cleaner since the pressure is released during the passage of the pulp through the feed nozzle and into the cleaner itself. In the case of a reverse hydrocyclone cleaner, bubbles small in diameter migrate quickly toward the central core area of the reverse cleaner, due to the large pressure gradient. Foam and water are removed at the base, at the rejects opening, and a deinked pulp is removed at the apex.

Forward and reverse cyclone cleaners are well known in the art, but their use in flotation type deinking is believed to be new, as described herein. The use of a hydrocyclone cleaner for deinking, in the context of this invention has particular merit in view of the relatively short residence time required, permitting the repulped material with hydrophobic contaminants to be quickly subjected to separation with low residence time, in order to take maximum advantage of the inherent hydrophobic nature of the contaminants. Examples of apparatus used in reverse cleaning include Braun U. S. Patent No. 3,912,579 and Braun et al. U. S.

Patent No. 3,557,956.

The invention may be characterized as a method or process for removing stickies and other hydrophobic contaminants. such as carbon and ink particles, from waste papers including the steps of performing an initial substantially dry repulping and dispersion operation to the waste paper to reduce the size and to disassociate the hydrophobic contaminants from the paper fibers, optionally storing such repulped and dispersed material for further processing, optionally subjecting the repulped and dispersed material to a screening to remove larger clumps therefrom, diluting said material with water to form a pumpable mixture suitable for flotation separation and immediately or as soon as practicable thereafter, subjecting said diluted material to a flotation type separation. In addition, the diluted repulped material may be subjected to water with dissolved air therein either as dilution water or as a downstream additive, and the combined streams applied as a slurry to the inlet of a reverse hydrocyclone cleaner so that the air expansion in the slurry entering the cleaner chambers forms bubbles for attachment to the hydrophobic components for delivery through the lightweight overflow port of the hydrocyclone cleaner.

The invention may be further characterized as a process of operating a flotation cell system for removing hydrophobic contaminants from a slurry of repulped paper stock in which the holding times in which the stock suspension is subjected to or mixed with water is reduced as far as practical to retain the maximum hydrophobicity of the hydrophobic particles, which may include stickies as well as carbon and ink particles, thereby to increase the efficiency of an existing system. In such a system, the time interval in which the repulped waste paper material is subjected to wet processing, in which the solid content in the slurry containing such material is less than 20%, may be reduced to as low as about five seconds prior to a flotation step.

In another aspect of the invention, hydrophobic components such as stickies and carbon, are separated from a paper stock slurry containing such components by a process including the steps of applying such slurry to the inlet of a hydrocyclone cleanser such as a reverse type cleaner and simultaneously applying with said slurry dissolved air in water either as part of the slurry or as an adjunct to the slurry, and permitting said air to expand as bubbles within the hydrocyclone to carry off as a foam or a foam rich stream such hydrophobic materials.

It is accordingly an important object of this invention to provide a method and apparatus by which old paper material can be repulped and the ink and/or stickies content subject to flotation separation in such a manner that the inherent hydrophobicity of the contaminants is preserved through the flotation step.

It is accordingly a further important object of the invention to provide methods of"flotation"employing hydrocyclone cleaners as a separating mechanism with air being injected at material going into the inlet either together with the pulp slurry or independently thereof, which material may have a substantial dissolved air component.

Another object of the invention is to provide systems and methods by which the input parameters including residence time, consistency, and/or chemicals added are optimized with the view of maintaining or enhancing the hydrophobic nature of the contaminants, to the end that bubble attachment is enhanced at the flotation step.

More particularly, it is an object of the invention to optimize the conditions that permit the removal of hydrophobic contaminants by taking such steps as to preserve the hydrophobic nature of such contaminants prior to flotation removal. These include optimizing the time of contact between the contaminant or wastepaper and water or other fluidizing material, optimizing the time of contact with surfactants, dispersants, and other chemical hydrophobic contaminants, selecting particular types and quantities of surface active or activating chemicals to minimize the reduction of hydrophobicity, and optimizing the reject removal in the flotation apparatus and method such as to maintain hydrophobicity of the contaminants at a level sufficient that air bubbles effectively become attached.

These and other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.

Brief Description of the Drawings Fig. 1 is a diagram of a system by which the invention may be practiced;

Fig. 2 is a further diagram showing another form of system by which the method may be practiced; and Fig. 3 illustrates diagramatically an aspect of the invention.

Detailed Description of the Preferred Embodiments bv Wav of Examples In order to obtain the greatest advantages of the concepts and methods of this invention, a wastepaper content to be repulped may be treated and handled in accordance with the teachings of co-pending allowed application Serial No.

08/651,812 filed May 25,1996 as a Continuation-In-Part of Serial No. 08/342,852 filed November 21,1994 to provide a relatively dry defibered pulp, now U. S. Patent No. 5,762,756 which is incorporated by reference. By"dry"I mean that the product will have a solids content greater than about 20% by weight and preferably between 20% and up to 80% or more by weight; effectively, a substantially dry product. Such a product may be defibered in a Hi-Con type pulper of Thermo Black Clawson Inc. as described in U. S. Patent 4,535,943. Alternatively, or in addition, a twin screw kneading device may be used as shown in U. S. Patent 4,993,649 in which screws rotated in opposite directions and are tapered in diameter, to affect simultaneous pulping kneading and dispersion. Other apparatus which may be employed for essentially dry pulping and dispersion include the Lemort"Betonniere"drum type pulper.

Example I With reference to Fig. 1, the dry pulper or kneader is illustrated at 10 and, as previously described, provides a relatively"dry"defibered pulp containing hydrophobic contaminants which may be subject to rapid dilution in a"zero"level stand pipe or induction pipe 15 where a controlled amount of water may be added from a water source 20. Sufficient water is added to provide a pumpable mixture in the range of about 4-5% solids and is delivered by a pump 21 through a coarse/fine screen 25. At the outlet side of the screen more water may be added to a dilution of about 2% consistency and this product is then delivered by a pump 26 to an air sparger or injector 30 directly to the inlet 32 of a reverse hydrocyclone cleaner 35.

The entire residence time of the dry pulp, containing hydrophobic contaminants, from the induction pipe 15 to the inlet of the reverse cyclone 35 may be in the order of as low as 5 seconds, although longer residence times may be required due to particular details of the equipment. In any event, the residence time is substantially less than the one to two hour or more resident time now conventionally employed in flotation systems. Thus, in this, the first step is the application of the dry defibered pulp from the defibering apparatus 10 to apparatus in which a sufficient amount of water may be added to provide a pumpable mix, such as 4-5% solids by weight.

The next step is to pump the resulting slurry through a suitable screen, such as a coarse/fine screen 25 for contaminant removal. Then, further water may be added to a consistency of about 2% and the suspension is applied through a sparger or air injector 30 to the inlet 32. Preferably the inlet consistency will be down to about 1% solids content. Alternatively, conventional flotation apparatus may be employed.

In the use of the reverse hydrocyclone 35, best efficiency may be obtained by taking a substantial liquid reject flow from the outlet port 40 at the base of the cleaner and providing downstream a second cyclone ore conventional flotation step to further concentrate the rejectable material and to recover fiber content.

Aeration of the feed flow to such a reverse cleaner may also be possible by the use of dissolved air only in relation to the relatively clear dilution water and by adding this air directly to the cleaner feed nozzle at 32 without the use of the air injector 30 but by pumping the pulp slurry and dissolved air components under pressure directly into the inlet 32. In this way, the dissolved air content may be contained in the dilution water immediately downstream of the filter 25.

Predominately foam will be separated through the port 40 at a low liquid and solids reject rate or alternatively, the foam may be discharged as a foam laden liquid stream. The latter may be advantageous when the addition of surfactants is minimized and under such conditions, it is possible that the foam particles may be weak, thereby releasing particles back into the fluid and it would therefore be advantageous to remove a fluid stream enriched with rejected particles rather than simply removing the foam content alone.

The example including Figure 1, whether the air is added by a sparger or directly to the inlet 32, is characterized by a minimum of dwell time from the dry pulping step, passing directly to a screening step and from there directly to a flotation step with a minimum of residence time. The residence time will be substantially less than the typical one to two hours or more residence time and may be as low as five seconds, although it is believed that quite satisfactory results can be obtained with a minimum of loss of hydrophobicity by times which are in the order of approximately five to approximately twenty minutes.

Example II In this example, illustrated in Fig. 2, wastepaper containing hydrophobic contaminants is repulped in a"dry"pulper 50 at an extremely high consistency such as about 50% with a minimum of water and chemicals. The pulper 50 may be the Hi-Con repulper as previously described or the twin screw kneader as previously described or the dry pulper of U. S. Patent 5,762,7566. Next, to reduce the requirement for chemicals, the repulped material may be deflaked in a deflaker 51.

This may be a"Micar"deflaker of Thermo Black Clawson Inc. Then the material is applied to coarse and fine screens 52, respectively. The dry screening is to remove coarse contaminants. At this time, the product may be stored in an optional holding bin 55.

In a further step, the dried and screened pulp is delivered from storage or the holding bin 55 in a rapid dilution step, in which water is added sufficient to make a pumpable suspension and delivered by a pump 57 to a conventional flotation tank 60 with a minimum of residence time, such as about five minutes or less. Air may be supplied by a conventional dissolved air applicator 62 at any particular stage in the process. The resulting slurry is subjected to flotation with a absolute minimum delay, with or without added preflotation chemical treatments. Thus, the contaminants are able to retain the highest possible degree of natural or inherent hydrophobicity to provide the maximum effective flotation separation.

Thus, the steps in this process could be described as follows: 1. Repulping in a"dry"pulper at about 50% consistency with water and a minimum of chemicals, and deflaking.

2. Optionally dry screening the resulting the pulp to remove coarse contaminants.

3. Optionally storing the dry defibered pulp in a holding bin.

4. Metering the dry defibered pulp for rapid dilution.

5. Pumping the resulting slurry with a minimum of delay to ink flotation, with or without added preflotation treatments.

Example III A relatively dry defibered pulp is mixed with water at point A of a process (Fig. 3), such as an existing process, where a slurry is formed which is sent to further process steps including de-ink flotation at point B of the process. The process is controlled so that the residence time between point A and point B is less than 5 minutes, and may be as low as 5 seconds.

Example IV Wastepaper, containing hydrophobic contaminants, is repulped with water containing predominantly only such chemicals which preserve the hydrophobicity of the particles, and only a minimum of chemicals which reduce the hydrophobicity of the particles, over time. The pulp is then mixed with air bubbles, to which particles may attach, for removal from the slurry by flotation.

Example V Repulping of wastepaper containing hydrophobic contaminants is performed under such conditions which preserve the hydrophobicity of the particle surfaces up to the flotation deinking step. Other chemicals, in the nature of conditioning agents including surfactants, collectors, dispersants, etc., which might reduce the hydrophobicity of the particles, are added immediately before or during the flotation, so that flotation is facilitated, while maintaining the time of exposure of the other chemicals to the hydrophobic particles.

Example VI Optimized conditions for removal of hydrophobic contaminants by preserving the hydrophobic nature of such contaminants prior to their removal.

Examples of various means to achieve removal are: --Optimize by reducing the time of contact between contaminant of wastepaper and water.

--Optimize by reducing the time of contact between surfactants, dispersants, etc., and hydrophobic contaminants.

--Select such types and quantities of surface active chemicals which minimize the reduction of hydrophobicity of contaminant surfaces.

--Optimize the means of reject removal flotation apparatus to permit operation of the flotation device in such a manner as to maintain hydrophobicity of contaminants at such a level that air bubbles effectively attach themselves to hydrophobic surfaces of the contaminants.

While the forms of apparatus herein described constitutes a preferred embodiment of this invention, it is to be understood that the invention is not limited to this precise forms of apparatus, and that changes may be made therein without departing from the scope of the invention which is defined in the appended claims.

What is claimed is: