Flotation deinking process

Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Waste paper or textile waste

Reexamination Certificate

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Reexamination Certificate




Around the world the use of the recycled fiber is now growing. A number of countries have already legislated that a certain part of the total paper production shall be constituted of recycled fiber. The methods which are used today for deinking are the flotation process and the washing process. The flotation process dominates and is the subject of the present invention.
The present invention is directed to the removal of inks from recycled paper products, i.e. de-inking. “Deinking” is the process of removing ink and other contaminants from waste paper and there are two main techniques in current use. “Flotation deinking” entails forming an aqueous suspension of waste paper pulp fibers, ink, and other non-cellulosic contaminants and then mixing air into the suspension. In the presence of certain additives, air bubbles selectively attach to ink particles and carry those particles to the surface of the aqueous suspension, thereby forming an ink rich froth. The froth is then removed leaving behind a relatively ink-free fiber slurry.
A deinking process can be divided in three different steps; 1) disintegration/dissolution of recycled fiber and release of the printing ink from the fibers, 2) dispersing printing ink in the water phase and 3) separation of the printing ink. These three steps are included in both the flotation process and the wash process. The method which one uses depends on the requirements and conditions which are found where the activity is located. The flotation process generally has been found to remove the greatest amount of printing ink from a recycled fiber suspension and to be the most cost effective.
The first step of flotation deinking is to solvate/saturate the fiber and disengage the printing ink. When this is accomplished the particles must be modified so that they obtain suitable physical and chemical characteristics so that afterwards they can be lifted to the surface and removed with help of air bubbles.
Some parameters are important to be consider at the flotation, such as the size of the particle, airflow (particularly the air:stock ratio) in the notation cell, the size of the air-bubbles, temperature, flotation time, pulp consistency (the concentration of the recycled fibers in the flotation cell), pH, the concentration of the chemicals as well as the degree of the hardness of the water.
The flotation process is carried out in such a manner that the recycled raw material of the fiber is passed through mechanical treatment in combination with associated chemicals. There are different methods depending upon the specific chemicals used, the concentration of those chemicals, and methods to process the raw material. The purpose of the mechanical treatment is that the paper be disintegrated into fibers that together with the chemicals result in an efficient removal of print ink. However, a too strong mechanical influence can cause undesirable damage to the fibers as well as ink smearing and/or ink re-attachment due to fibers contacting one another.
Then comes filtering and dewatering, whereby the particle impurities are separated, followed by a purification step, generally by means of a vortex cleaner, where particles are separated. In a dispersing step the discharge of printing ink continues through mechanical treatment and chemical enhancement. Printing ink and any remaining small particles are separated subsequently in the flotation step. Different flotation techniques exist and in the more contemporary ones, one can float all small particles by pressurizing the flotation cell. Small impurities are separated through washing and to increase the brightness of the pulp, bleaching is performed through use of hydrogen peroxide, hydrosulfite, and the addition of conventional optical bleaches. Optical bleaches enhance the contrast between the ink and the paper background causing the paper to look brighter and to enhance ink colors.
Flotation deinking processes are especially useful in removing hydrophobic inks with particle sizes larger than about 10 &mgr;m. The additives used in such processes are generally specialty surfactants or fatty acids which are intended to agglomerate the relatively finer ink particles to increase removal efficiency in the flotation stage. The presence of additives which over-disperse the ink particles rather than agglomerate them is considered detrimental to the effectiveness of the flotation stage.
Newspapers, magazines and other printed media have been recycled for many years. Recently the need to recycle paper has increased significantly and will likely continue to increase in the future in view of environmental concerns and legislative action. To reclaim fibers from printed material, a deinking process is required to remove the ink and other contaminants. Deinking of waste paper has become increasingly more difficult because of changes in the printing techniques being used and the wide variety of printing inks. As a result a slurry of recycled waste paper contains a complex mixture of inks, resin binders, fillers, and the like, which must be removed.
The conventional industry chemical formulations for flotation deinking for many years have included a fatty acid or fatty acid soap. See, for example, U.S. Pat. Nos. 4,964,949 and 4,483,741. However, fatty acids and fatty acid soaps have a number of problems associated with them, e.g. high dosage rates (typically about 16 pounds/ton of waste paper but as high as 30 pounds/ton), relatively poor foamability thus causing the high dosage rates, and the general need for high levels of water hardness to achieve acceptable performance. The hard water often leads to handling problems as well as scale and deposit buildup in mill equipment. More recently, non-ionic surfactants have been developed for use in deinking systems.
The use of various types of cationic materials in compositions for removal of inks from waste paper has been disclosed in several earlier patents. For example, polyoxyalkylene compounds containing amine or quaternary ammonium groups have been disclosed in such as U.S. Pat. Nos. 4,483,741, 4,605,773, JP 59 137587, DE 3,928,599, DE 4,007,598, and DE 4,007,597. Similar formulations which are claimed to be effective specifically for deinking of paper printed with flexographic inks have been disclosed in DE 4,007,596, WO 90 05806 and EP 478505. Surfactants such as the ones described in the preceding patents may be regarded as materials in which a single substance contains both non-ionic and cationic moieties. None of these prior art compositions, however, have been found capable of adequately removing the very fine hydrophilic inks in a flotation deinking stage or a combined flotation and wash deinking system.
U.S. Pat. No. 5,736,622 discloses a synthetic collector which consists of a polyester based on a polyalkylene glycol and a di or tricarboxylic acid. The patent goes on to improve on such a deinking agent by adding into the polymerization mixture a saturated fatty acid with 12-18 carbon atoms, and controlling the molecular weight to be between 3,000 and 10,000.
While a polyester has been found to be helpful in improving brightness and ink removal efficiency, the performance has not been found to be sustainable under continuous real world mill operations due to adverse changes in the foam profile, i.e. the foam begins to build with time and will not collapse at the desired rate to efficiently remove the ink. Further, the extended foam lifetime has been found to result in rejects backup which results in float cell leveling and overflow/surging problems. The addition of conventional defoamers, e.g. silica/silicone and ethylene oxide/propylene oxide surfactants and the like, have not corrected the problem. Rather they have been found to persist in the system and penalize generation of the desired de-inking foam profile. Ink re-attachment has been observed with use of the defoamers.
Accordingly, it is an object of the present invention to provide a method of deinking printed media whereby improved foam control allows enhanced continuous running c


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