In the past deinking of waste paper was mainly practiced to obtain cheap raw material for newsprint production. In North America, washing system was practiced. In Europe, flotation system, which used alkali, sodium silicate and hydrogen peroxide in repulping of waste paper, was used. This kind of deinking did not normally comprise any bleaching stage within the main deinking system, but possibly a post-bleaching with sodium dithionite. During the past decade deinked pulp (DIP) has found increasingly place as a raw material for higher grade papers such as supercalandered (SC) and light weight coated (LWC) papers, which are used in magazines, catalogues etc., and in the production of fine papers, which are essentially woodfree and in tissue production. This has meant higher brightness target with high cleanness requirements. Flotation deinking and flotation deinking with two flotation stages comprising also an alkaline hydrogen peroxide bleaching stage has become more and more popular.
In order to detach the ink from the repulped waste paper alkali is added to swell the fibers and hydrogen peroxide is added for preventing alkali darkening. Sodium silicate (water glass) is added to buffer the system, to stabilize peroxide, to collect the ink and to prevent the redeposition of ink particles on fibers. In the deinking of woodfree (WF) papers, such as office papers, hydrogen peroxide and sodium silicate (water glass) is seldom used. In order to collect the disintegrated ink from the repulped waste paper, fatty acids as their calcium soaps were used as ink collectors. The calcium was separately added in the past, but since the current waste papers contain a lot of calcium containing pigments and fillers, a separate addition of calcium salts are seldom any more needed. In addition hard water and the closure of water circulations have contributed to the content of calcium.
At the moment the most used method for deinking waste paper is the flotation deinking. The configuration of the system depends much on the raw material and on the end use of the deinked pulp.
The most used system covers the following units. An alkaline repulping of waste paper in the presence of alkali and silicate for buffering and ink dispersion and in the presence of hydrogen peroxide for reducing alkali darkening of pulp. A surface active agent is also in general added into the pulping equipment. This is still quite often a fatty acid, which needs to be in form of a calcium salt in order to collect the disintegrated ink particles, though non-ionic synthetic surfactants are also used. After the repulping, the cleaning and screening section are following. There is, after this unit, the first flotation unit, flotation I, where the surfactant can also be added. After cleaning and screening a kneader or disperger is used to detach more residual ink. If higher brightness is needed for the deinked pulp, e.g. if the deinked pulp is not used for newsprint, but e.g. as raw material for SC or LWC paper or if mixed office paper is used as raw material, an alkaline hydrogen peroxide bleaching stage is followed, which is followed by the flotation II. This can then be followed by a reductive sodium dithionite bleaching stage, which is often practiced when deinked pulp is targeted for newsprint production and the process does not include the peroxide bleaching stage. The deinking can also be carried out at only one flotation stage and the process can also include washing stages, which are very common when deinked pulp is used for tissue production, when the low ash content in the tissue paper is important to the tissue properties. When colored papers are deinked the last bleaching stage can also be carried out by a reductive sodium formamidine sulfinic acid (FAS) for effective color reduction. If the production of DIP is connected to a paper mill the water introduced to the DIP process comes as so called white water from the paper mill. The DIP mills having two flotation units comprise normally two recycling water loops, which nowadays most often also comprise a microflotation unit for cleaning the waters to be recycled.
The conventional deinking process can also be such one that the repulping is carried out without hydrogen peroxide, but after the repulping of the wastepaper and the cleaning and screening section there is a disperger/kneader or soaking tower. The bleaching of the pulp is either carried out in the disperger or in the soaking tower, which is followed by only one flotation unit. This system is very much practiced when old newsprints or when old newsprint and magazines are deinked in order to produce pulp for newsprint production.
For every special application the DIP has a certain brightness target, but important parameters in the deinking is the residual ink content, speck count or area and yield. For the production efficiency as high yields as possible with target pulp properties is the goal in order to obtain good profitability. This means that as little as possible loss of fines and pigments and fillers present in the raw material waste paper, should occur. This is not always the case, when tissue is made, since the different tissues have specific low ash requirements. A part of this is achieved by selecting low ash containing waste papers such as low ash containing office paper as the raw material. Since the papers varies, the DIP process for tissue production, always comprise one or more washing stages, which will decrease the yield by removing fillers and pigments.
Also in deinking office papers, normally the so-called mixed office waste (MOW), which can contain at maximum 10-15% wood containing grades and other fiber materials such as different board materials, the low ash content is also an aim, since the mineral fillers and pigments can adversely affect the high brightness requirements and other pulp properties. Also, in deinking old news paper (ONP) and old magazine paper (OMP) raw materials for production of deinked pulp for production of SC and LWC paper, low ash content can also be the aim, since the ash can adversely affect strength properties.
A lot of different equipment, which have not been mentioned above can be used in the deinking processes.
Generally following chemicals are used in deinking:
Alkali is used in repulping in order to swell the fibers and thus to alleviate the detachment of ink from the fibers.
Sodium silicate (water glass) is used for buffering the system, for ink dispersion from the fibers, ink agglomeration, preventing ink redeposition and preventing decomposition of the used peroxide.
Hydrogen peroxide to counteract the darkening caused by alkali. Hydrogen peroxide is used also in a separate bleaching stage.
A flotation agent, i.e. a surfactant to collect the ink.
A foaming agent, if needed, to improve the function of the ink collector in the separation of foam.
Calcium ions to obtain collector properties, when fatty acids are used as ink collectors.
A chelating agent to prevent the decomposition of hydrogen peroxide by transition metal ions.
A biocide or an anticatalaze agent to inhibit the decomposition of hydrogen peroxide by catalaze enzyme.
The normal terms used, when making deinking performance studies are:
ERIC (Effective Residual Ink Concentration) in ppm.
Hyperwash, a special efficient washing, in which the loosely bound ink can be eliminated and the measurements done for the pulp describes the pure deinked pulp.
Dirt or speck count and area or image analysis depicting the contamination by residual ink etc.
Brightness or R 457 value.
Yield of the pulp based on the used amount of waste paper.
Problems associated with the use of sodium silicate and the current status of the technology.
As described earlier water glass is abundantly used in deinking of waste papers due to the many beneficial properties. Water glass causes also many problems, since it gives very easily deposits, especially in the presence of calcium in the fiber line and the carry-over of silicate with the water-containing pulp from the fiber line can cause runnability problems in the paper machine. Therefore it would be advantageous to replace the silicate, but this has not yet succeeded.
The surfactants used as ink collectors can be divided into the following classes:
Anionic surfactants including soaps, linear alkylbenzene sulphonates (LAS), fatty alcohol sulphates (FAS) and fatty alcohol ether sulphates (FEAS).
Cationic surfactants including quaternary ammonium compounds (QAC).
Ampholytic surfactants including ampholytic surfactants (AMPH).
Non-ionic surfactants including fatty alcohol ethoxylates (FAEO) and propoxylates (FAPO) and combinations thereof (FAEPO), nonylphenol ethoxylates (NPEO), and other surfactants, such as alkoxylated polyesters, fatty acid ethoxylates and propoxylates, ethoxylated vegetable oils etc.
Soap is still the dominant surfactant at least when old newsprint (ONP) and old magazines (OMG) are deinked, but it requires calcium ions in order to function as ink collector, but the calcium soap has among other things the drawback of causing deposit problems and negatively affecting the sheet properties, when deinked pulp is used in paper making. All the other surfactants have also some drawbacks, but they are used in special applications, e.g. in deinking of mixed office waste (MOW), in tissue production etc and in some countries and combined with certain kind of equipment when the certain benefits can be obtained and the normal drawbacks, such as the reduced yield of fillers is not of importance.
The use of sodium salt poly-(α-hydroxyacrylic acid) (PHAS) and the corresponding polylactone in deinking of ONP and OMG mixture is disclosed in U.S. Pat. No. 4,347,099 (Solvay, 1976). The deinking has been carried out in the presence of a fatty acid at specific hardness and also in the presence of a foaming agent.
It is also set forth in U.S. Pat. No. 4,347,099 that the PHAS and the ink collector can totally be omitted and a polyacrylic acid can be used in instead of PHAS. The only properties, which have been measured, are brightness and hydrogen peroxide consumption, in addition to the pH. The deinking is a complex process and also other parameters such as the yield, the amount of residual ink and speck area should be measured in order to assess the performance of the deinking process. It can also be noticed that different deinking systems are specific for the used waste papers and sometimes also for inks used in printing the paper. In addition to the required pulp properties, the yield in a deinking process is a very important factor, since the yield will define a part of the cost efficiency of the process. When ONP based only on old newsprints is deinked, a yield of 90-95% can be obtained. When waste papers containing fillers and mineral pigments, i.e. OMG, are used together with ONP, an average yield of 85% is obtained, while making deinked pulp for tissue production from this kind of raw materials the yield can be as low as 50%. In deinking of wood free grades, the yield is not always very important, e.g. when the deinked pulp is used for making uncoated fine paper. One tries to guarantee good properties of the end product paper and therefore the filler and pigments content, measured as ash content should be relatively low. This means that the yield can be below 80%, depending on the original filler and pigment content of the used raw material waste paper.