New approach combines latest fiber kneading,
high-efficiency washing technologies
Historically, washing deinking has been considered a simple and economical process that can produce only lower quality DIP (deinked pulp) with high dirt count. But today this is no longer true. A growing number of companies around the world are currently using the latest generation kneading technology, paired with advanced washing technology, to obtain significant quality, operating, energy and environmental advantages compared with conventional deinked pulp (DIP) processes such as flotation deinking.
But at this time advanced kneading technology paired with the latest washing system approaches are not being used in North America, at least not on a full commercial scale. Hiroki Nanko, president of Insight Technology International LLC, in Macon, Ga., USA, is, however, in the process of changing that. Nanko has set up a pilot plant operation near Macon to demonstrate the performance of a Taizen kneader (named New Taizen) and a high efficiency pulp washer known as the vertical washer (named Vertical Z). Both machines are manufactured by Taizen Co. Ltd. (Japan). The technology Taizen has packaged into a three-stage system (conventional pulper/kneader/vertical washer) is especially effective for producing DIP to be used in making tissue products.
All deinking systems consist of two main steps: detachment of ink from fibers; and removal of the detached ink. According to Nanko, the Taizen kneader achieves the first step and the vertical washer can achieve the second step at a high level. “It is a very simple system that can produce high quality DIP,” Nanko says.
The main source of wastepaper for tissue is printing and writing (P&W) paper. Since P&W paper production has been reduced significantly in recent years, the quality of recovered paper has declined. Instead, a variety of smaller quantities and grades of paper are now being processed by deinking plants.
“For any deinking system, the most important thing is that the separation of ink and fiber is complete. If ink detachment is not complete, then washing itself cannot help. Technically it is getting more challenging these days because the quality of recovered papers is declining and various grades of papers printed by different types of
inks and toner needs to be processed by a single DIP plant. Taizen’s washing deinking system is very forgiving of this situation,” Nanko explains.
“In the Taizen kneader,” he continues, “fibers are exposed to gentle but intensive rubbing motion for a long time (about 10 min). Friction between the fibers breaks almost all of the fiber-ink bonds in a recycled stock. The resulting ink particles are small, which makes them more easily removed in the subsequent vertical washer stage, unlike with flotation deinking, which is only efficient for ink particles larger than 10μm. Mechanical treatment by existing devices such as high-speed dispersers, deflakers and some kneaders generally cannot achieve fiber-ink detachment as well as the Taizen kneader because of shorter duration time and less intensive rubbing motion between fibers,” he adds.
“In the washing stage, not only the detached ink, but also the ash (fillers and coatings) and fines need to be removed. The ash content in today’s recovered paper has been increased due to the increased use of coated paper and LWC. For tissue application, low ash content is a requirement since ash interferes with fiber-to-fiber bonding and slows drainage. Fines also need to be removed to improve drainage on the tissue machine. There are technologies, such as the DNT washer, that can achieve this, but cause significant fiber loss in the process. The vertical washer can remove ink, ash and fines without losing long fibers,” Nanko emphasizes.
TAIZEN WASHING DEINKING SYSTEM
The Taizen deinking system (Figure 1) begins with a conventional pulper that prepares the wastepaper for treatment in the gentle kneader. The pulper disperses fiber at medium consistency and is followed by detrashing devices that take out basic “garbage” that comes in with the wastepaper.
After detrashing , the pulp feed is thickened to about 25-30 percent solids using a cylinder press (twin roll press) and then fed into the kneader unit. The cylinder press is also a Taizen unit, but Nanko says that other equipment such as a screw press can be used for this function. According to Taizen, the cylinder press has much lower energy consumption than a typical screw press and it does not lose fibers.
The Taizen kneader is designed to maximize friction between the fibers, with a kneading consistency of 25-30 percent. Rubbing action of the fibers separates ink from the fiber surface and breaks it down to a small particle size. As noted earlier, gentle and low-speed (100 rpm) kneading at high consistency does not cause fiber cutting or fibrillation. Very little or no freeness reduction happens
Unlike conventional kneaders, the New Taizen kneader consists of three cylinders with a single shaft (see photo and Figure 2). Pulp stock is pushed forward at the first half of each cylinder by the feed screw, and the stock is exposed to gentle and low-speed kneading (100 rpm) at the latter half by the kneading blades.
Each cylinder is equipped with a single motor and a reducer. Kneading speed is constant for all three cylinders (100 rpm). The intensity of kneading is controlled by the air cylinder pressure to the baffle located at the end of each cylinder, and that of the three cylinders can be controlled independently. Feed stock goes through all of the three cylinders in about 10 minutes. Overall, energy usage of the kneader system is “pretty good,” according to Nanko, at 60-80 kwh.
Although the design operating consistency in the kneader is 25-30 percent, the first and second cylinders have drain function to push the consistency back up to the optimum range in case it falls below 25 percent.
Due to the high intensity of friction between fibers, pulp temperature rises quickly to around 80oC without injecting steam. This high temperature softens the fiber and accelerates ink separation.
“The Taizen kneader is an excellent high consistency mixer. Taking advantage of the high pulp temperature and good mixing effect, pulp bleaching can be done in the kneader. Bleaching chemicals can be introduced to each cylinder (sodium hydroxide, hydrogen peroxide, FAS, etc.). And then after it comes out, it can sit in the tower for a while to continue the bleaching process.
Brightness improvement in the kneader is a significant advantage for the reduction of capital investment cost,” Nanko says.
“Although the Taizen kneader introduces high intensity of friction among fibers, it does not cause cutting or shortening of the fibers, which is very significant for tissue making. Kneading does not generate fines or cause freeness reduction,” Nanko points out.
Prolonged rubbing of the fibers also makes them more conformable and pliable, improving fiber bonding. Kneaded pulp shows good wet and dry bond strength, Nanko adds, explaining that low paper breakage on the paper machine and low fiber linting have been experienced with pulps made with the Taizen system. It is known that recycled fibers are stiff because of the hornification effect, he adds, and thus will be tissue products made with them. However, tissue products made with DIP produced by the Taizen kneader are known as “soft tissue.”
“In addition to deinking, Taizen discovered a very useful side effect of kneading. Micro-stickies agglomerate and become removable by actions of the kneader stage,” Nanko notes. The kneading process at high consistency raises the temperature high enough to soften and activate the surface of stickies, but not high enough to melt the stickies, unlike a hot disperser. The stickies encounter other stickies during kneading. Since the stickies have better affinity to the stickies themselves than wet fibers, the activated stickies tend to stick to one another to make larger particles by the snowball effect.
Nanko examined the agglomerated stickies formed by the Taizen kneader under the scanning electron microscope (SEM). The photo on the left (page 16) shows agglomerated sticky balls formed by kneading. The middle photo shows a very large agglomerated sticky ball that includes fibers and the photo on the right shows agglomerated micro-stickies covered by filler particles.
Generally, the balls of stickies include some fibers in them, and the surface of sticky balls are sometimes covered by filler particles, which make them less active (less sticky). The sticky balls are big enough to be easily separated by a fine screen. “Removal of stickies from DIP is still a big, challenging issue for recycled tissue mills. The Taizen kneader can provide a practical solution for this problem,” Nanko says.
Pulp from the kneader stage is sent to a reaction tower to complete bleaching. Then, it is sent to a dilution tank followed by low density cleaners and two pressure screens for removal of stickies and other large particles. Then, it goes to the vertical washer.
Nanko points out that there is somewhat of a functional paradox with traditional pulp washing devices. Washing at low pulp consistency improves deinking, but fiber loss increases. Washing at high consistency can minimize fiber loss, but hinders deinking. Existing pulp washers-thickeners tend to lose long fibers when the ink removal effect is maximized.
Pulp from the kneader unit is fed into the vertical washer, which basically removes ink particles, ash and fines. After being processed in the vertical washer, pulp cleanness goes up dramatically, the fiber becomes whiter and there is significantly less ink. A DNT washer, Nanko notes, would do basically the same thing, but would lose significant amounts of long fiber in the process. The vertical washer does not lose long fiber, only fines, he emphasizes.
Energy consumption of the vertical washer is similar to a DNT washer, but fiber-economy-wise, the vertical washer is considerably more efficient. Maintenance cost is also comparatively low for a vertical washer, compared with a DNT washer, which has a high speed rotating wire that needs to be periodically replaced. The drum screen of a vertical washer is exceptionally durable. The oldest vertical washer has been used in a production line for 11 years and it is still running without replacement or repair of the drum. The working system of the screen inside the vertical washer is highly automated, so that it doesn’t have to be manually stoked at all. The only maintenance is occasional replacement of bearings and seals.
The vertical washer that Nanko has in Macon is a 50 tpd unit. It weighs about three tons. It has a relatively small footprint (1.3 x 1.3 meters) at 3.1 meters tall and 1.2 meters in diameter. This machine is a functional unit that can be installed in a pulp mill
“If a company is interested, I can bring the kneader (it has wheels on the bottom) for testing at a mill site, as well as the washer and related equipment. It can be loaded into a trailer and hauled. A loading dock is available at the pilot plant site. I’m currently exploring shipping the machines for mill testing overseas,” Nanko says.
Figure 4 shows the structure of the vertical washer. It consists of three components – a rotating drum screen, an agitator located inside of the drum and two shower systems to prevent clogging of holes on the drum.
Pulp slurry is fed into the bottom of the drum through the bottom holes of the agitator shaft (Figure 4). The slurry moves up slowly in the drum while being vigorously mixed by the mixing blades. The drum screen has holes of 0.3 mm diameter. Due to the vertical arrangement of the drum screen, a fiber mat formation on the screen surface by gravity can be avoided. In addition, turbulence created by the mixing blades removes fibers covering the inner surface of the drum. The constantly exposed holes of the screen improves removal rate of suspended particulates including ink particles, fillers and fines.
The drum and the shaft rotate in opposite directions, generating turbulence in the slurry to improve washing efficiency. The rotating drum generates centrifugal forces that contribute to dewatering and thickening of the pulp slurry. Washed pulp is recovered at 4-10 percent consistency. Suspended particulates are pushed out through the holes on the drum screen by the centrifugal force of the rotating drum screen, while the long fibers are retained inside of the drum.
There are two shower systems for cleaning the outer surface of the drum. One is a high pressure oscillating shower that is operated continuously to prevent clogging of the holes. The other is a fixed fan shower designed to remove slime from the drum.
The Taizen kneader and Taizen vertical washer are both well-seasoned technologies for the commercial production of DIP worldwide, except North America. The largest kneader is 250 TPD capacity and the largest vertical washer is 100 TPD capacity. About 150 kneaders and 90 washers are currently in operation worldwide.
The world is moving toward sustainability these days, and improvements in sustainability of recycled pulp production is a very important challenge for the tissue industry, Nanko says. “Due to the shortage of high quality wastepaper, the tissue industry is beginning to use more virgin pulp. However, if we have deinking technologies that can utilize low quality recycled papers and can produce higher quality DIP without losing long fiber in the deinking processes, the tissue industry can save large amounts of wastepaper that otherwise goes to landfills. The tissue industry should consider new technologies from the sustainability viewpoint as well as from the fiber economy aspect,” he emphasizes.
Figure 1. Flow Diagram of
Taizen Deinking System.
Figure 2. Structure of New Taizen Kneader.
Figure 3 is a cut-away drawing showing the inside workings of the kneader. As the blade rotates, one area with inserted stator blades has less space and another area has more space, repeatedly compressing and then releasing the pulp. Also, the angles of the blades are not the same, being designed to push forward and/or push back the material. This repeated compressing and releasing of the pulp forces the fibers to rub against each other under pressure to break away
ink particles, improve fiber pliability (flexibility and conformability) and
generate process heat.
Figure 3. Inside of the Kneader.
New Taizen, Kneader Developed by
Taizen Engineering Co., Japan.
The vertical washer removes ink particles, ash, and fines without losing long fiber.
Figure 4. Structure and mechanism
of the vertical washer.
Sheet brightness (l-r) after pulper
(52%), after kneader (41%), and
after the vertical washer (85%)
More About Kneader Technology at Tissue Forum
The TAPPI Tissue Forum at PaperCon, May 16-18, 2016, in Cincinnati, Ohio, USA, will feature a report by Hiroki Nanko of Insight Technologies International (whose work is discussed in this article) on kneader technologies used to process alternative fibers for tissue and towel production. Nanko’s presentation is set for Session TF2 from 3:30-5:00 p.m. on May 16.
Exploring a new Alkaline Peroxide Mechanical Pulping (APMP) system based on the Taizen kneader technology, Nanko’s presentation focuses on pulping of alternative, non-wood fibers to be used in the production of tissue products for both the at-home and away-from-home market sectors. The presentation examines a wide variety of non-wood plant materials that can be pulped, including wheat straw, rice straw, bagasse, bamboo, kenaf, Arundo donax, Miscanthus, switch grass, flax, oil palm EFB, etc., with this mechanical pulping system.
Once in the kneader, this material is mechanically treated in the Taizen three-cylinder structure, with chemicals, typically 3 percent NaOH and 3 percent H2O2, added. Heat is generated due to the friction of the material during long kneading time, and the pulp temperature rises to more than 90° C. Softening of middle lamella lignin between fibers by the heat and the chemicals allows fibers to be separated by kneading. Hydrogen peroxide treatment makes the pulp whiter when it exits the kneader. Additional refiner treatment of the pulp, followed by kneading, breaks up the residual shives into individual fibers.
Gentle chemi-mechanical treatment of the Taizen system keeps the fibers long with no “cutting,” so that the pulp produced is suitable for making tissue.