What is bentonite in pulp?

Bentonite, also known as bentonite, is a mineral stone containing natural aluminum silicate. When it swells with water, it can absorb 8- 15 times of its own water content, and is widely used in adhesives, adsorbents and emulsifiers. Bentonite particles can be used to produce high-grade paper products, such as writing printing paper, newsprint, paper in contact with food and cardboard.

Application and development prospect of bentonite in paper industry

Lu Zhaoqing Zhang Meiyun

Abstract: China is rich in bentonite mineral resources. Bentonite has special physical and chemical properties and has many uses in paper production. This paper mainly summarizes the application status and development prospect of bentonite in papermaking industry, focusing on the application of bentonite as a process aid in pulp and paper production, such as retention and drainage, sewage treatment, resin control, paper color development and waste paper deinking.

Key words: bentonite; Paper industry; Application; prospect

Bentonite is a kind of fine clay with montmorillonite as the main component. China is rich in bentonite mineral resources, and all 23 provinces (regions) in China have bentonite minerals. It is predicted that the resources will be more than 7 billion tons. By the end of 2005, China has proven reserves of nearly 5 billion tons, ranking first in the world. There are many kinds of bentonite, including calcium bentonite and sodium bentonite. Although China is rich in bentonite reserves, the research on the development and application of bentonite is not extensive enough, which restricts the transformation of China's expanding resource advantage to economic advantage to some extent. Pulping and papermaking is a complex industry, involving many physical and chemical reactions, and various inorganic and organic additives need to be added to optimize the process. Bentonite, as an excellent additive, is widely used in pulp and paper production. This paper mainly introduces the application of bentonite in retention and drainage, sewage treatment, resin control, waste paper deinking and paper processing.

Composition and physicochemical properties of bentonite

Montmorillonite is the main mineral of bentonite and a layered aluminosilicate mineral containing water. Montmorillonite crystal is composed of many parallel crystal layers, and each crystal layer is composed of upper and lower silicon-oxygen tetrahedron and middle aluminum-oxygen octahedron layer. Its chemical molecular formula is a12O3.4sio2.nh2o, and the crystal layer contains water molecules and exchangeable cations. Al3+ in octahedral vacancies is often replaced by low-priced Mg2+ and Fe2+. Si4+ in tetrahedral gap is replaced by A 13+. Because low-priced cations replace high-priced cations, additional negative electricity prices are generated in montmorillonite structure. In order to maintain the electrical neutrality between montmorillonite layers, there are cations such as Na+, Ca2+ and Mg2+ with larger radius besides water molecules. These cations are exchangeable, so montmorillonite has a series of valuable characteristics for paper industry, such as ion exchange, water absorption, swelling, thixotropy, cohesiveness and adsorption.

2. Application of bentonite in pulp and paper industry

2. Application of1bentonite in retention and drainage aid

With the emergence of higher speed and more complicated paper machines and various chemical additives, closed circulation of white water, increased filler load and increased consumption of secondary fibers, papermaking technology has made great progress. Increase the retention rate of fine fibers and fillers; At the same time, improving the drainage condition of paper machine can greatly improve the economic benefit. And can improve some properties of paper. In addition, the increase of paper machine speed will inevitably lead to greater shear force during paper forming, which requires that the flocs formed must have certain shear resistance, and traditional process improvement and mechanical methods can no longer meet this requirement. In order to achieve these goals, we must develop cleaner, more effective and more economical retention and drainage AIDS.

Hydrocol system is a new type of multi-component particle retention and drainage aid system developed by United Colloid Company in Britain, which is composed of modified bentonite and cationic polymer. The retention mechanism is that high molecular weight polymer is added to the slurry, and the floc formed is cut by the mixing pump and slurry screening machine, and the modified bentonite is agglomerated in the slurry to form a unique agglomerate structure. Even in white water, a strong re-agglomeration effect can be seen.

The typical bentonite particles in the retention and drainage aid system are layered, with a layer thickness of 0.9nm and a surface area of about 300nm× 100n m, with 5 ~ 7 layers stacked. Its specific surface area during hydration is large, about 700 ~ 800 m2/g, and the surface of bentonite sheet is negatively charged and the edge is positively charged, so it is easy to adsorb colloidal substances. When bentonite is hydrolyzed in water, cations form negatively charged particles in the form of sodium ions. This negatively charged particle is not affected by pH value, and it also performs well in acid and alkali. It is suitable for acidic and neutral papermaking in all fields of machine-made paper and paperboard, and can obtain high retention rate and high drainage effect that can not be obtained by single polymer in the past. The above characteristics of bentonite are widely used in papermaking process, such as retention and drainage aid and white water recovery and treatment.

The following is a production example of using Hydrocol particle retention system in white water sealing system: a factory producing GCC corrugated board in Montana, Canada used Hydrocol bentonite retention and drainage system after white water sealing, the discharge of papermaking wastewater decreased from 6000m3/d to zero, the retention rate increased from 85% to 92%, and the energy was saved by 5% after white water reuse.

2.2 Application of bentonite in sizing

Bentonite can form a well-dispersed suspension in the water phase, so it can be used as a stabilizer for some additives in the water phase. The recent American patent reported the application of bentonite in sizing, adding reactive sizing agent to bentonite suspension, thus using less or even no surfactant. The slurry is influenced by bentonite dispersed in water and has good dispersibility and stability. This sizing suspension can be used for sizing inside or outside the papermaking process. Using bentonite instead of surfactant has good physical properties, which improves chemical stability, reduces the occurrence of hydrolysis and reduces the adverse effects of surfactant on papermaking process. When used as internal sizing, bentonite can also play the role of particle retention and drainage system. The bentonite used is sodium-based, lithium-based or sodium salt activated bentonite.

2.3 Application of Bentonite in Wastewater Treatment

The application advantage of bentonite in pulp and paper industry wastewater treatment lies in its strong adsorption and decoloration ability. The purpose of reducing COD and decolorizing is achieved by adsorbing pollutants in wastewater. Bentonite used for wastewater treatment usually needs organic modification to enhance its adsorption capacity of organic pollutants. Commonly used organic modifiers are organic amine cationic surfactants, such as cetylpyridinium chloride (CPC), cetyltrimethyl ammonium bromide (CTMBA) and tetradecyl pyridinium chloride (TPC). Modified bentonite has a strong affinity for organic matter, which can not only be adsorbed on the surface of particles, but also enter between bentonite layers to form bentonite-organic complex. In addition, the modified bentonite has a good adsorption effect on heavy metal ions.

In the process of alkaline papermaking or kraft pulping, cooking wastewater is called black liquor, which is dark in color and smelly. The black liquor from 1t pulp production is 15 ~ 20m3, and 30% of the total water pollution in China comes from papermaking black liquor, which is seriously polluted. The treatment of papermaking black liquor with bentonite has been reported abroad for a long time, but there are few applications and related documents in China. Sun Jiashou of Wuhan Institute of Chemical Technology prepared organic cross-linked bentonite by silicon cross-linking and organic modification, and discussed the adsorption performance of organic cross-linked bentonite for COD in the water after acidification and flocculation of black liquor. The results show that when pH is 4.5, TBC dosage is 24g/LH2O, and stirring adsorption is carried out at room temperature for 60min, the adsorption efficiency of COD is 665,438 0.5%, and the adsorption capacity is 549.3 mg COD/g.. In addition, bentonite can be used in deinking wastewater treatment and sludge dewatering with polymer flocculant. Yang Jiuyi and others introduced a method to treat papermaking black liquor wastewater by using bentonite to prepare flocculant. Through orthogonal test, the optimum technological conditions were selected, and the removal rates of chromaticity and COD reached 90% and 70% respectively. Shao Hong et al. synthesized a series of modified bentonite with Ni-Zr inorganic pillared and Ni-Zr organic pillared from sodium bentonite, and compared the treatment effects of different modified bentonite on papermaking wastewater. The results show that organic pillared bentonite has the best treatment effect on papermaking wastewater, and the removal rates of COD, chromaticity and turbidity are 77.8%, 94% and 97.4% respectively.

2.4 Application of Bentonite in Resin Control

In the pulping process, a large number of water-soluble polymers and colloids are released into the water during the production process. Wood colloid and resin are easy to deposit on the surface of equipment and products, which will lead to production problems and paper defects. The methods of resin control are mainly divided into physical control and chemical control. Among them, chemical control method is the most commonly used and effective resin control method at present. It uses some resin control agents to make the resin adhere to the fiber surface or disperse stably in the slurry system, thus avoiding the deposition of the resin on the equipment surface. Modified organobentonite is also an effective resin control agent because of its large specific surface area and organic affinity. It can adsorb hydrophobic colloidal resin in the system, reduce the surface energy of the resin, and make the resin lose its unique viscosity, thus inhibiting the adhesion, coalescence and deposition of the resin. Adding 0.5% ~ 1% bentonite to the pulp can effectively adsorb these impurities and prevent them from aggregation. Bentonite can not only adsorb colloidal resin, but also adhere to agglomerated resin particles, thus reducing the viscosity of resin. However, when there is shear force in the system, the agglomerated resin particles may re-emerge on the fresh surface and tend to agglomerate and deposit, so it is very important to choose the adding position of bentonite reasonably.

2.5 Application of Bentonite in Waste Paper Deinking

In the waste paper recycling system, bentonite (used alone or mixed with talcum powder) can adsorb and bind ink and stickies to fibers, thus minimizing the tendency of ink and stickies to deposit on various parts of the paper machine. In the recycling of waste paper, there are spots visible to the naked eye in the finished paper, which is the result of poor dispersion of ink particles because of "difficult dispersion". Particles are usually flaky, and the thickness varies from a few microns (small particles) to 10μm (large particles). In the pulping process of waste paper, spots are controlled by mechanical dispersion, particles are broken mechanically, and spots become invisible by reducing particle size. These particles are not removed but remain in the pulp, so the pulp turns gray and the whiteness is greatly reduced. If bentonite is added to waste paper ingredients, the formation of spots can be prevented. Bentonite can reduce the size of ink particles and remove them from pulp by washing, or aggregate small ink particles into large-size particles, which can be effectively removed by flotation or centrifugal purification, thus improving the whiteness of pulp. Bentonite has played many roles in the whole process, such as adsorption, dispersion, precipitation control and anti-redeposition.

2.6 Application of Bentonite in Pigment Coating Processing

Coatings used to produce coated paper are mainly composed of pigments, adhesives and additives. Among the additives used, there is a rheological control agent, whose function is to adjust the rheological properties of coatings to meet the requirements of production technology and avoid product quality problems caused by poor rheological properties of coatings. When montmorillonite is dispersed in water phase, the surface hydration and the repulsion of interlayer electric double layer will increase the interlayer spacing and expand the volume of bentonite, which will thicken bentonite in water phase system, so bentonite can be used as rheological control agent for paper coating. At the same time, because bentonite has a regular layered structure, it is also beneficial to improve the coverage and smoothness of the coating, thus playing a certain role as a pigment.

A patent reports the process of producing coated paper with bentonite as part of pigment, and its products are especially suitable for photo printing and offset printing after calendering.

2.7 Application of bentonite as filler

As mentioned above, bentonite has a strong hydration, so it can form a fine flaky structure in a colloidal state, absorb a large number of positively charged electrolytes, and can be combined with negatively charged fibers, thus leaving bentonite on the fibers. At the same time, bentonite also plays the role of paper filler. In addition, bentonite has the characteristics of "high quality and low price", which can be applied to the production of low-basis weight paper, and a higher filling amount can be obtained without reducing the strength of paper, especially the tearing degree. The application research of a research institute in Hangzhou shows that modified bentonite can be used as a filler for low whiteness paper and paperboard (bentonite itself is not high whiteness), showing strong adsorption and high dispersion, high retention rate and little influence on paper strength. At the same time, because the modified bentonite can adsorb fine substances such as fine fibers, it can reduce the concentration of white water discharged from paper mills and reduce the pollution of discharged sewage to water quality.

2.8 Application of Bentonite in Paper Color Development

Natural bentonite treated with hydrochloric acid, sulfuric acid or phosphoric acid is called acidic clay. Compared with bentonite, acid clay has significantly increased specific surface area, high adsorption performance and catalytic activity. Pressure-sensitive copy paper used for fax paper, invoices and computer records uses the chemical reaction between acid clay and white (colorless) dye to realize chemical color development.

Bentonite, as the color developer of force-sensitive pigment in carbonless copy paper, has been produced and used for a long time in Britain, Germany and Japan for acid activation treatment of bentonite products in carbonless copy paper. Zhejiang Institute of Geology and Mineral Resources used Qiushan high-quality bentonite to produce bleaching soil for carbonless copy paper. This product has been used by Beijing Paper Research Institute and other units, and it is considered that it can completely replace imported products.

Taking three-layer carbonless copy paper as an example, gelatin and Arabic gum microcapsules containing pigments are wrapped in the adhesive used on the back of the upper paper and the middle paper, and acidic clay is attached to the surfaces of the middle paper and the lower paper as a color developer. Pigments in paper are protected by microcapsules and will not develop by themselves after overlapping. However, when the paper surface is pressed with a pen tip or typewriter needle, the microcapsules will break and the dye will overflow and contact the coating surface. Acidic clay developer has adsorption force and can provide protons. After the leuco dye obtains protons, the lactone ring breaks to form a polarized ion structure to realize color development.

3 abstract

In a word, bentonite is widely used as a particle retention and drainage aid system, but there is not much research and application in other aspects, which needs further development. Bentonite has excellent properties. According to the analysis of its physical and chemical characteristics, it has great potential application value in many links of pulp and paper making. With the development, modification and deep processing of bentonite, its application field will be further expanded. At present, many domestic enterprises still rely on imported bentonite. Bentonite reserves in China rank first in the world and are widely distributed. It is of great significance for the paper industry to make full use of this unique advantage and independently develop high-quality bentonite products.