Fundamentals of De-inking Technology of Recycled Paper
The demand of paper has been continuously increasing at a pace much faster than the availability of fibers from the natural sources. Recycling of waste paper, after its intendment use, has been found to be more economical and eco – friendly. Without recycling, the fiber supply from the world’s natural sources shall not be sufficient to keep up with the demand. Recycling efficiency can be increased further by choosing recovered paper by grade and reusing high value papers. The reuse of the paper fibers is essentially dependent on their proper processing and de-inking. Only after the ink has been removed from the fibers, the fibers can once again be used to make a good paper. The technology advancement has given us opportunity to use waste paper in manufacturing specialty papers too, like writing and printing grades. For manufacturing writing and printing grades, efficient de-inking of the waste paper is an essential operation of the paper making process. Flotation deinking operation is found to be the most efficient process now a days. The most important factor, which will decide the growth rate of paper industry in the coming years, is the availability of suitable raw materials economically on sustained basis.
Advantages 0f Recycling:
• Environmental considerations
• Decreasing availability of conventional raw materials
• Lower energy requirements
• Improved technology available for reuse of secondary fibers, to obtain better quality products
• Freedom from complete dependence on market pulp.
• Reliable source of pulp in times of market pulp shortage.
• Price usually favorable in comparison with that of corresponding grades of market pulp.
• The kinds of de-inked pulp suitable for use in printing papers usually impart special properties to the finished papers compared with papers made from wood pulp, such as:
- Increased opacity
- Less curling tendency
- Less fuzziness
- Better formation
- Better retention of size and fillers
- Softer and less tiny character
2. Waste Paper Grades
The type, cost, and quality of wastepaper are critically important to the design and operation of a de-inking plant. Waste paper grades are used to:
1. Classify the various types of waste paper.
2. Describe the type of waste paper included in a particular grade.
3. Specify general quality requirements for each grade.
4. Provide a basis for establishing the market value of each wastepaper grade.
Though a major segment of the Indian industry is utilizing the recovered paper for papermaking, yet the industry is facing serious problems in processing due to inconsistent quality. The major factor is improper and unorganized collection and distribution system still prevailing in the country. Due to lack of an Indian grading system, at source grading / sorting is not practiced; as a result most of the paper is recovered in mixed form and not\ in grades. This results in high percentage of prohibitive material and out throws which adversely affect the quality of recovered paper stock.
Moisture Content: The average moisture content of waste paper in bales/loose or in bags should be limited to 10% as determined by standard methods. The buyer has a right to request for adjustment of weight proportionate to excess moisture content on oven dry basis.
Out Throws: Non- cellulosic material which cannot be used as paper making raw material. Example: Rags, tapes, pet/glass bottle, iron/plastic strapping, polybags, plastic cards etc.
Presence of unusable grade of cellulosic raw material makes it unsuitable to use in that specific grade of paper/paper board.
1. Presence of mechanical fibre in wood free fibre.
2. Presence of unbleached fibre.
3. Presence of wet strength paper in regular grade of paper.
4. Presence of clay coated paper in uncoated paper.
5. Presence of carbon paper in office based.
6. Presence of wax coated box in old corrugated boxes.
7. Aluminum coated paper in box board cuttings/duplex cutting/wide cuttings.
8. All such similar cases with special reference to specific grade.
The market value of wastepaper grades consumed in de-inking plants is influenced by the following main characteristics:
1. The optical properties of the fibers contained in the wastepaper (brightness, color, etc.).
2. The types of fibers contained in the wastepaper (mechanical fibers, chemical fibers)
- The types of materials applied to the papers contained in the wastepaper (clay based coatings, laser inks etc.).
- The presence and amount of various other types of waste paper in the market, and their costs.
- The presence and amount of contaminant materials in the wastepaper.
3. Processes and their Equipments
Normally, the equipment used in Secondary fiber processing is of the same type found in other pulp and paper mills processing virgin fiber. Some equipment are specifically modified for the use of secondary fiber, or use different mesh wire and screens. The refining of recycled paper will require more gentle treatment, as the fiber has once before been processed for its earlier specific use. Secondary fiber washing system operates at lower consistencies without forming tight mats for optimum effectiveness. The material being washed off the fiber consists more of particulate suspended solids rather than dissolved solids as in the washing of chemical pulp.
Secondary fiber processing requires a multiplicity of cleaning and screening equipments, in different sequences, combinations and sizes, as per specific needs. The screening equipments work on the principle of size and shape for removal of contraries. Combinations of screens with different sizes of slots and holes are often used as standard equipments in all the secondary fiber based mills.
All the three types of centricleaners i.e. forward, reverse and high – density are normally used in the secondary fiber processing, in different stages, which work on the principle of difference in specific gravity of the constituent fiber, debris and contaminants for separation / cleaning.
Each mill has its own configuration of different process equipments depending on the raw material used and to make desired products.
4. De-inking Operation
The function of the pulper in a de-inking operation is to defiber the paper and to detach the ink particles from the fibers, while retaining the contraries (undesirable materials) large enough to be removed by centricleaners and screens in subsequent stages. The flotation unit has often been referred to as the “heart” of the de-inking system. It is reasonable then to think of the pulper as the “brain” of the system. If the pulper does not work properly, or if the chemistry of the chemicals added is unbalanced, the batch has little chance of success. Normally the pulper has provisions for supplying heat to obtain the desired temperature at the desired mixing speed and consistency.
High consistency batch pulping normally uses a helical rotor, which is very much larger in relation to the pulper tub than a low consistency rotor. The low rotor speed relative to low consistency pulping reduces the cutting of plastic and other contaminants, but shearing forces are high, since the velocity difference between the stock and the rotor is high. Mechanical forces are low and when no screen plate is fitted mechanical forces are very low. Discharge from the pulper can be through a screen plate, or direct, with no screening. When discharge is through a screen, contaminants retained in the pulper are flushed out and dewatered; extracted water can be returned to pulper.
Advantages claimed for high consistency pulping when compared with low-consistency include:
– Reduced contaminant breakdown, leading to better contaminant removal.
– More complete ink removal. However, the intense fiber interactions result in reduced ink particles size and so the brightness from high consistency pulping is usually less than for low consistency pulping. Nevertheless, after washing, when most comparisons are made, brightness of high consistency pulped stocks is higher, due to better ink removal during the washing which occurs as the test sheets are made.
High-consistency continuous pulping-drum pulpers:
Although the discharge from drum pulpers is in the range 3-5%, defibering occurs at 15-20%. Pulping drums are divided into two sections, the wetting and defibering section and a screening section. These can be either one complete unit, or two separate drums, rotating in the same or opposite directions.
There are no mechanical forces and no high-shear zones; wetting and defibering is only by the tumbling and falling action within the drum. The second stage is screening, and stock consistency is reduced to 3-5% in the perforated screen zone. Fibers and small fiber
flakes are washed through the perforations, which are 4-10 mm in diameter. All materials not reduced to below the perforation size in the first zone are rejected out of the end of the drum. The very gentle defibering action is evident by the condition of the rejects; sheets of plastics are rejected as a whole without rips.
Temperature and pH each have a significant effect on both the speed of wetting and defibering as well as on ink and other contaminants dispersion. Higher pulping temperatures also increase ink and other contaminant dispersion, and the trend is towards lower pulping temperatures to reduce stickies problems. pH also has a significant effect on yield loss- dissolution of fiber components, and probably dispersion of ink increases with a higher pH.
There are four basic methods for removal of ink particles & other contaminants from recycled fibers:
(a) Washing (b) Flotation (c) Cleaning (d) Screening
Washing efficiency is best at a range of 1-10 microns. Flotation efficiency is high through the next range of 10 -150 microns. Cleaning equipment works best at 100 -1,000 microns and screening at 1,000 microns and above.
4.2 Effect of Different Chemicals on the Waste Paper
As discussed above a large number of chemicals are used in the processing of waste paper. Each such chemical contributes differently with different functions as detailed below -
Sodium Hydroxide (NaOH)
Sodium hydroxide is used not only to adjust the pH to the alkaline region, but to saponify and /or hydrolyze the ink resins. The alkaline environment is often reported to “swell “the fibers. This term is more descriptive than reality. At the pH conventionally used for pulping; 9.5 -11.0, the fibers would take up some water and become more flexible, rather than puff up like cellulose balloons. The addition of caustic soda to Mechanical pulp, furnishes will cause the pulp to yellow and darken. This is the phenomenon that is often referred to as “alkali darkening”. The problem with alkali darkening is only of concern with wood containing furnishes. Higher pH (=11) can be used with wood free furnishes with no Alkali darkening.
Hydrogen Peroxide (H2O2)
Hydrogen peroxide is usually added to prevent Yellowness, to strip the color and to increase the brightness. The decomposition of peroxide can be reduced by the addition of “stabilizing “agents such as chelants and Sodium silicate. Hydrogen peroxide is also used as post bleaching agent. The balance between such peroxide should be added in the pulper versus how much in the bleaching stage must be optimized for each furnish. It should be remembered that the peroxide is added to the pulper simply to affect the formation of chromophores created by the alkaline pH.
DTPA (diethylene triamine penta aceticacid) is the most commonly used chelant but EDTA (ethylene diamine tetra acetic acid) is also used. The role of chelant is to form soluble complexes with heavy metal ions .The complexates prevent these ions from decomposing the hydrogen peroxide.
Sodium Silicate (Na SiO)
The Sodium silicate most commonly used in de-inking mills is at 41.6O Baume solution of sodium metaselicate which contains roughly equal amounts of SiO2 and Na2O. Silicate aids in de- inking through an ink dispersant action or by preventing the ink from redeposting on the fiber surface. The anti –redeposition effect is what made silicate popular in laundry soaps. The dirt or soil was emulsified and prevented from sticking back on the clean wash. The fact that silicate is a source of alkalinity and will affect the pulper chemistry, increasing the silicate will increase the pH and this may call for a reduction in the sodium hydroxide addition rate.
Surfactants / Active Chemicals
The term surfactant is derived from their function as surface active agents. “Surfactant” is a catch all term that covers, uses like dispersants, collector, wetting agents, displectors, anti – redeposition aids and the like. Surfactants that are used for de- inking will have two principal components – a hydrophilic and a hydrophobic component. During flotation, the hydrophobic end will associate with the ink oil and dirt while the hydrophilic end will remain in the water. Some of the most common surfactants used in de-inking are the Eo /Po (ethylene oxide /propylene oxide) copolymers .The hydrophilic (water liking) is the ethylene oxide and the hydrophobic (water hating) end is the propylene oxide end. Active chemicals (proprietary de- inking chemicals combining functions of dispersant and collector) help the ink and dirt particles removal in suspension, which have been freed during the pulping operation. It helps their removal during washing and flotation.
4.3. De-inking by Flotation Cell
Flotation de-inking is a selective separation process that utilizes air to separate ink particles from a properly slushed pulp suspension. As the air bubbles rise through the flotation cell, ink particles become attached to the bubbles and are carried to the surface of the cell. Flotation chemicals are added to the pulp to make the ink particles hydrophobic and increase the probability of flotation. Inky foam forms on the surface of the cell and is removed, while the fiber remains in the pulp and is accepted. The flotation cell operation is normally carried out at around 0.9% to 1.5% consistency. A number of properties are measured in the finished pulp obtained after flotation de-inking. Most important of them are ISO Brightness, ERIC950, Yield, Opacity and De-inkability factor. ERIC950 and De-inkability factor have been defined below:-
ERIC 950 values have been determined for repulped pulp and de-inked pulp obtained after inked foam removal from the flotation cell. The basic purpose of ERIC 950 option is to determine how much residual ink remains in the sample of de- inked paper. This is accomplished by measuring the reflectance in the infrared region of spectrum (950 nm) and manipulating these reflectances via Kubelka Munk analysis until the Effective Residual Ink Concentration (ERIC) is computed.
The efficiency of a de-inking operation may be defined as the ratio of amount of ink removed, to the amount of ink present before de-inking. De-inkability factor, for any process where ink particles have been removed, is defined as -
D= E – E X 100%
D =De-inkability factor, %
E = ERIC value of the sample sheet after pulping before ink removal in flotation cell.
E =ERIC value after flotation de-inking
E = ERIC value without the presence of ink particles (blank)
Similarly, the de-inkability factor can also be estimated based on brightness values for all the three types of pulp, as for ERIC values.
4.4. Effects of Variables on De-inking Operation Performance
The amount of ink removal increase as we increase the temperature and consistency up to the optimum conditions and then it decreases as we exceed the higher limits. With the increase in active chemicals, the yield, opacity and ISO Brightness increases. It has been observed that with the increase of surfactants/ Active chemicals; during pulping operations the ERIC value decreases but after flotation the ERIC value increases relatively. This leads to net decrease in de-inkability with the increase of active chemicals. The reason for the same is that with the increase of surfactants relatively more debris, loading material and ink shall first get collected in pulping system and then go out with the foam in flotation process, and with the increase in concentration of surfactants/ active chemicals up to optimum dosages, we get improved de-inkability from the process.
An effort has been made to discuss in brief the relative advantages of using recycled paper, different recycling processes and equipments used with De-inking operation in some detail. The important effect of process variables has also been discussed. The importance of adding different de-inking chemicals during pulping and slushing operation and flotation de-inking operation have been discussed in brief, to highlight the importance of related operating and performance parameters. The importance of De-inkability factor, ERIC950, Opacity, Yield and ISO Brightness to evaluate de-inking operation had also been discussed briefly.