In 1808, a fourdrinier papermaking machine improved by the British John Gamble and Brian Donkin was officially born, which marked the modernization of papermaking machinery. Since then, mechanical engineers and papermaking technicians around the world have continued to explore and practice, promoting the continuous improvement of papermaking equipment and paper quality.
With the continuous improvement of paper machine operating parameters and paper performance requirements, papermaking machinery and papermaking technology have developed rapidly. Among them, wet end technology is particularly critical, especially core components such as headbox and dewatering elements are crucial in matching the operating speed and paper forming quality.
This article takes the fourdrinier papermaking machine as an example to introduce the function, structure and material evolution of ceramic dewatering elements.
In the wire section of the papermaking machine, ceramic dewatering elements are used to preliminarily dehydrate the pulp, so that the water in the pulp is quickly separated to form a wet paper web. Its excellent wear resistance and smooth surface can effectively reduce damage to the forming wire, improve the quality and strength of the paper, while reducing energy consumption and reducing wear of the wire section and the press section.
Material evolution
Early dewatering elements were mainly made of high-density polyethylene (HDPE). However, with the continuous increase in paper machine speed and the development of material science, ceramic materials have gradually become the mainstream choice due to their excellent wear resistance and low friction performance. Commonly used ceramic materials include alumina, zirconium oxide, silicon nitride, silicon carbide and their modified materials.
Modern ceramic dewatering elements usually adopt a double-layer structure: the ceramic layer is in direct contact with the forming mesh, and the supporting material below is fiberglass. The two are bonded by glue or mechanical structure to form a stable scraper.
Different ceramic dehydration elements are used in different stages of the dehydration mechanism
- Gravity dehydration zone
- Low vacuum dehydration zone
- High vacuum dehydration zone
- Pressure dehydration zone
Gravity dewatering zone
The ceramic dewatering elements in the gravity dewatering zone mainly include forming plates and gravity scrapers.
Forming plate
The principle of papermaking is to evenly distribute plant fibers in a suspension with water as the main component, and then remove most of the water to form a wet paper web. After the pulp is ejected from the headbox, it is sprayed onto the forming net at a certain angle. Such a strong impact will cause deformation and damage to the forming net.
Therefore, as the first dewatering element, the forming plate, not only plays the role of supporting the forming net and receiving the pulp, but also changes the arrangement direction of the plant fibers from the original messy distribution to the distribution in the MD direction of the paper machine. A large amount of suspension remains on the forming net. As the forming net moves forward, the initial dehydration and forming are completed.
The width of the forming plate is 100-350mm, the width of the rear scraper is 65-80mm, and the spacing between the two is 50-70mm. The ceramic surface angle is 0°; the arrangement is usually: 1 forming plate + 2-5 rear scrapers.
Forming box
Gravity scraper
The water and a small amount of plant fiber in the slurry pass through the forming net under the action of gravity and are scraped off from the lower surface of the forming net by the front angle of the gravity scraper. At the same time, some water and fiber are sucked down from the vacuum wedge area formed on the ceramic surface and the lower surface of the forming net, and are scraped off by a scraper behind.
Gravity scraper width: 50~80mm, horizontal width 10-15mm, bevel angle α is 0°~5°, bevel width 40~70mm; increasing the length and bevel of the bevel can increase the dehydration of the scraper. Bevel angle α is a key parameter affecting dehydration efficiency. Generally, in the production process, the angles are arranged from small to large to achieve the ideal dehydration effect. Sometimes, gravity water hanging boards can be arranged at intervals of large and small angles to increase the pulp jumping effect and improve the uniformity of the paper width. The arrangement method is: 3~7 strips.
Gravity box
Low vacuum dehydration zone
The ceramic dehydration elements in the low vacuum dehydration zone mainly include leveling box and low vacuum wipers.
Leveling box
Leveling boxes are usually composed of two types of wipers, including ceramic box and ultra-high molecular polyethylene (HDPE) wipers, which are arranged at intervals.
The ceramic box is in direct contact with the forming mesh, while the HDPE wiper is lower than the mesh surface by a certain value to form up and down pulses.
During the operation of the paper machine, due to the height difference between the scrapers, the pulp suspension will produce vertical pulses, effectively preventing the situation where the pulp finely blocks the forming mesh, improving the dehydration effect of the forming mesh, and improving the uniformity of the paper web; when reaching this position, the water content in the suspension decreases sharply, and dehydration requires the assistance of vacuum water legs, with a vacuum degree of -5~-12 kpa.
The width of the ceramic scraper is: 35~50mm, the width of the uniform scraper is: 50~80mm, the height adjustment range is 1-4mm, and the arrangement is generally: 7~13.
Leveling box
Low vacuum box
Low vacuum box is an important component used in the dehydration process of papermaking machine. It is located in the middle and rear part of the fourdrinier paper machine.
It uses the suction force of the vacuum pump to quickly dehydrate the paper sheet under the action of low vacuum suction force, thereby improving the dryness of the paper sheet and increasing the dehydration efficiency. The vacuum degree is -15~-20 kpa.
The width of the ceramic wiper is: 35~50mm
The arrangement is: 7~13 strips.
Low vacuum box
High vacuum dehydration area
The high vacuum water absorption panel is forced to dehydrate by vacuum pump suction, so that the paper sheet transitions from low vacuum dehydration to high vacuum rapid dehydration, thereby accelerating the drying of the paper sheet, enhancing the dehydration effect and the forming of the paper sheet.
The panel is equipped with adjustment strips at both ends to adjust the width of the suction port of the paper sheet. The adjustment block is also equipped with a lubrication device to reduce the friction coefficient through water lubrication and increase the service life of the net.
The panel structure is reasonably designed and is a sealed structure. The panel is processed as a whole, using special machine tools and special processing technology to ensure high overall processing accuracy, smooth surface and high finish.
Due to its wear resistance and self-lubricating properties, the high vacuum water absorption panel can reduce the wear of the paper sheet during the dehydration process, improve the quality of the paper sheet and the operating efficiency of the paper machine.
The vacuum degree is -25~-60 kpa.
The width of the ceramic strip is: 15~25mm
The arrangement is: one chamber, two chambers, and three chambers.
one+two+three chambers
Pressing and dehydration area
The felt scraper or felt panel has 2 or 3 scrapers combined to form a single or double slit structure, with a slit width of 10~15mm; there is also a single scraper designed as an eccentric structure, which adjusts the slit width according to the wetness of the felt to increase or decrease the dehydration amount;
The ceramic edge chamfer is large, and the common chamfer is R3~R5, which can not only extend the service life of the felt, but also prevent the felt from shedding.
felt suction box
Summary
As an important part of the wet section of the Fourdrinier papermaking machine, ceramic dewatering elements play a key role in the initial dewatering of pulp and the quality of paper sheet formation. From the early high-density polyethylene materials to the high-performance ceramic materials widely used today, dewatering elements have been continuously improved in terms of wear resistance, dewatering efficiency and forming effect.
Ceramic dewatering elements will continue to develop towards higher wear resistance, better surface finish and intelligence to further improve the operating efficiency and paper quality of papermaking machines, and provide strong support for the efficient and sustainable development of the modern papermaking industry.
In the near future, with the development of artificial intelligence, automatic control, and functional material science, paper will no longer be “paper”, and will be given more new, strange and special functions; the dewatering machine of the papermaking machine will be transformed from the current physical mechanical dewatering to a more intelligent, more efficient, more environmentally friendly and more energy-saving diversified process.
