Discussion on Application of Waste Paper Deinking Technology

Summary:Usage of waste paper; basic processing technology of waste paper; development of waste paper deinking technology; factors affecting deinking

First, the use of waste paper

Waste paper began as a renewable resource for the paper industry at the beginning of the 20th century. It is also called "secondary fiber," and its usage and its proportion in papermaking raw materials have increased year by year. Today, due to people's demands for improving the environment, protecting forests, saving energy and raw materials, and increasing the demand for papermaking costs, together with increasingly stringent environmental protection requirements, rising prices of paper-making materials and energy pressure, waste paper as a renewable resource, It has attracted the attention of governments and companies around the world. Compared with the pulping of directly using plant fiber raw materials, waste paper pulping can save a lot of energy, save plant fiber raw materials, reduce product costs, and save equipment investment and reduce environmental damage. The use of waste paper to make paper can save 50% of water, reduce the corresponding discharge of sewage, save energy by 60-70%, save 2/3 of construction investment, save the amount of chemical raw materials such as sodium hydroxide, reduce environmental pollution, and reduce air pollution by 66%. -70%; reduction of 40% in biological oxygen demand; 25% reduction in suspended solids in sewage; 70% reduction in solid waste. From this we can see that the recycling of waste paper has significant social and economic benefits.

Second, the basic treatment of waste paper use

The waste paper can be used for batters, linings, core pastes, corrugated base papers and high-strength corrugated base papers, household papers, newsprint papers, and various cultural papers (especially medium and low-grade cultural papers) of various paperboards. The use of waste paper for deinking in China started late, but it has developed rapidly in recent years. The waste paper processing technology, high-concentration crushing, desorption, washing, pulp screening, flotation, heat dispersion and bleaching technologies and equipment are becoming more and more sophisticated and more and more advanced. The processing capacity of domestic waste paper production lines ranges from 30t/d to the current 100-150t/d, and there are a number of 300t/d and 500t/d production lines put into operation. How to rationally and scientifically select waste paper deinking process and equipment is the key to the application of waste paper deinking pulp and creating good economic benefits.

Third, the progress of waste paper deinking technology

The increasing demand for waste paper has made low-quality domestic mixed waste paper and office mixed waste paper become the main waste paper sources for extensive collection and recycling; the ever-expanding variety of waste paper specifications, and the increase in the number of recycled papers, has enabled the collection of The quality of waste paper is getting lower. The progress of society requires the use of low-quality waste paper to produce high-quality paper; the rapid development of modern printing technology and various new inks such as UV-curable inks, laser printing inks, and electrophotographic inks, water-based printing Ink, etc., are also significantly different in composition and properties from conventional inks. All of these requirements are forcing people to continuously improve, innovate and improve waste paper processing technology. Many researches have been conducted in this area at home and abroad, and there have been many new developments.

(A) Enzymatic deinking

The traditional method of deinking waste paper uses a chemical method, which requires a large amount of alkali, water glass, and industrial soap. The ink on the paper is emulsified under the action of strong alkali and dispersant to be separated. However, with the rapid development of printing technology at home and abroad, the type of waste paper has undergone great changes, and the proportion of laser and photocopying waste paper accounts for more and more, and the formula of printing ink is also constantly changing. Many factories have only to break up large-particle inks by mechanically breaking them up and then removing them by flotation. The use of enzyme deinking method has obvious advantages: it can reduce energy consumption, reduce environmental pollution, deinking efficiency is higher than chemical deinking, enzyme deinking pulp has good physical strength, high whiteness, low dust, less residual ink, and filtration Water is good.

Current deinking enzymes include esterases, esterases, pectinases, amylases, hemicellulases, cellulases, and lignin-degrading enzymes. In actual use, cellulase and hemicellulase are the main components. Compared with the conventional alkaline deinking, the COD produced in the enzymatic deinking process is 20%-30% lower than the alkaline process.

(B) Ultrasonic deinking

Ultrasonic deinking method usually uses liquid whistle ultrasonic generator to produce acoustic wave with frequency of 20-60KHZ. The periodic pressure change produces cavitation, longitudinal vibration occurs, and tiny bubbles with diameter of 0.1-0.2mm are formed when the bubbles are affected. When the ultrasonic wave acts, it can constantly absorb the energy generated by the continuous compression and expansion cycle of the ultrasonic wave, so that its size fluctuates or its diameter increases. When the bubbles increase, they absorb the maximum energy, increase rapidly, and then burst. When these bubbles burst (time is within 1us), its internal temperature reaches 5500°C. Due to the small volume of the bubbles, this heat energy disappears very quickly. At any time, the liquid remains at a temperature close to the environment, and the broken bubbles will Energy is transferred to secondary fibers near it. This energy has two effects: one is to relax the ink particles and remove them from the fiber surface; the second is to break up the ink particles by themselves. The deinking process of waste paper is finally achieved.

Ultrasonic deinking method has its advantages: it can use less or no chemicals, reduce costs, and reduce waste water pollution; high removal rate of high gloss ink, good deinking effect; DIP breaking factor, crack length, whiteness and other indicators Obviously shows improvement, but tearing factor does not decrease.

(3) Agglomeration and deinking

Agglomeration deinking is similar to conventional chemical processing combined with mechanical action to separate the ink from the fibers, but in a different manner. The method does not disintegrate or disperse the ink to make it smaller. Instead, the ink particles are aggregated into spherical particles with a density of more than 400 μm, and the ink size and relative density are increased. The ink is then cleaned by centrifugation and removed by a combination of screening methods. . Usually when decomposing the separated fibers, 1% of the hydrocarbon oil agglomeration agent is added to make the peeled ink integrate particles with a particle size of 1 mm to 1 cm, and the agglomeration agent adsorbs the ink particles around it through the liquid bridge, and the temperature during the process. And the PH value has little effect on the ink removal effect of this method, and the processing time is short, generally within 15min, no need to add other chemicals. The ink removal rate is 95%-97%. However, the high degree of sizing and the waste paper present in cationic starches have a poor deinking effect.

(4) Steam explosion deinking method

Waste paper has been de-inked by steam explosion for more than 30 years, previously limited by intermittent operations. In recent years, due to the development of new steam blasting technology, production capacity has been greatly improved. After de-inking by decomposing steam, the ink particles are crushed to a small size (<75um), and then de-inked pulp is obtained after purification, screening, and washing. The process can effectively recover low-grade waste paper, and can solve some difficult problems in the recovery of waste paper, such as plastic hot melt, wax, rosin, etc. can be effectively treated. And laser, electrostatic copy ink processing is very effective. The technology has the advantages of simple process, low operating cost, investment saving, energy saving and environmental protection.

(5) Solvent deinking

The main feature of the solvent deinking method is that it does not use water or only a small amount of water. When removing laser printing paper and xerographic paper ink, trichloroethylene and ethane are used as solvent, and the heated plastic structure is used to dissolve and fix the plastic structure of the laser ink. After cooling, these cured products become hard, and then undergo screening and removal. The slag and other processes remove the ink while recovering 90-99% of the solvent. This method can be used for deinking old newspapers, old paper, and mixed office waste paper.

(6) Enzyme-Ultrasound Synergistic Deinking

Enzymatic treatment reduces the consumption of chemicals in the deinking process of waste paper. However, due to the limitations of biochemical reactions, enzyme treatment takes a long time. Ultrasonic treatment can cause periodical compression and expansion in the liquid, causing the fibers to vibrate strongly and rub against each other, increasing the activity of the cellulase, thereby accelerating the removal of the ink particles. The enzyme-ultrasound synergistic effect has improved the deinking efficiency of color offset newsprint, and the whiteness of deinked pulp has increased by 5%. The fiber thickness and length are reduced to some extent, but the paper strength has little effect.

Fourth, the factors that affect the deinking of waste paper

The waste paper recycling process is a very complicated system project. At present, the use of chemical-mechanical deinking process in China's domestic small and medium-sized paper mills is still relatively common. From the process analysis, waste paper sorting, dissociation, aging, deinking and other processes and equipment rationality have a greater impact on the deinking of waste paper.

(I) Influence of deinking method

At present, domestic deinking methods mainly include washing method and flotation method. The former methods of disassociation and maturation of the two methods are basically the same, except that the method for separation of pulp and ink particles is not the same. The washing method uses a dehydration method to separate ink and impurities from paper fibers. Since fresh water is replaced by washing in turns, the degree of cleanliness is high and the whiteness of the pulp is high, but at the same time, microfibers and ash are easily washed away, which affects the slurry rate. The law applies to the production of tissue paper and low ash paper grades.

Flotation deinking is based on the principle of flotation in the mining industry. According to the different wettability of fibers, fillers, and inks, flotation machines are used to remove the ink (poor wettability) from the surface of the slurry and fibers and fillers remain. In the slurry, thus reaching the upper separation. The flotation method is a deinking method in which a gas-liquid-solid surface participates in the deinking method. The deinking method has a small loss of fine fibers, a high ash content and a high slurry yield, and is suitable for paperboard type paper.
(b) Effect of deinking agent

The deinking agent plays a key role in the deinking process. In the deinking process, what deinking agent is selected and how the deinking agent performance directly affects the deinking effect.

Anionic and non-ionic surfactants are the main components of the deinking agent. Non-ionic surfactants are used in flotation. The deinking effect is good and the whiteness of the pulp is high. However, in the deinking of the washing method, the deinking effect of the ether type nonionic surfactant is better.

(III) Effect of Deinking Temperature, Time, and Deinking Agent Consumption

In the deinking process, deinking time, temperature, and the amount of deinking agent are important factors affecting the deinking effect. In general, the longer the time, the higher the temperature, the greater the amount of the deinking effect is better, but from the cost, efficiency and other comprehensive factors to consider, should be a reasonable selection of the best process conditions and parameters.

(4) Influence of printing methods and ink composition

The rapid development of printing technology and the appearance of inks with different compositions have greatly improved the color, luster and firmness of prints, but at the same time, it has also increased the difficulty of deinking waste papers. Therefore, a comprehensive understanding of ink composition and curing mechanism has been achieved. , printing methods, etc., can effectively remove the ink. Here are some common printing methods to talk about the waste paper deinking method used in different printing methods.

Letterpress printing: This method is commonly used in the printing of newspapers, magazines and kraft paper bags. The ink is mainly carbon ink, dispersed in the carrier of hydrocarbon oil, and dried on the paper by means of absorption, volatilization and precipitation. Relief paper of letterpress printing is relatively easy to deink, only need to add 1%-2% of active deinking agent, use the washing method to remove the ink.

Offset printing: This method is applied to the printing of glossy magazines, books, and artwork. Ink contains water-repellent pigments. It is strong in color and insoluble in water and alcohols. The ink contains alkyd resin and drying oil. Offset printing ink resin is difficult to disperse, but adding silicate and surface active deinking agent for deinking, while using flotation and washing combined deinking can achieve better results.

Flexible printing: This method is an improvement of letterpress printing. Flexible printing inks are fast-drying, low-viscosity inks that use a water-based carrier as a carrier and are dried by evaporation and absorption. The flotation method can be used for deinking. The dispersion collection method is used first, and the deinked pulp with good quality can be obtained with secondary washing.

Waste paper deinking pulping is a complex system engineering. We need to further explore and explore the process characteristics of the entire process, and design and develop newer deinking technologies to make deinked pulps become low cost and good quality. Sub-fiber raw materials, to create a good social and economic benefits.