A few decades ago, pulp quality in a paper mill could only be judged in a laboratory, using a number of standard testing methods such as drainability (freeness), percent of unseparated fibers (Sommerville shive) and size classification by a series of screens (Bauer-McNett). To evaluate pulp strength, you need to follow a laborious process of making pulp handsheets and performing destructive tests on them. By the time all these tests were complete, the pulp was often already made into paper ready to be shipped. If downward trends in pulp quality were not detected in time, multiple rolls of paper had to be rejected or downgraded because quality trends were not available in time for a correction to be made.
Today, the story is much different. The evolution of online testing, automated sampling equipment and robotic testers for offline testing of both pulp and paper, combined with data historians, sophisticated control systems and emerging artificial intelligence techniques, have given the industry powerful tools to gain a much tighter control of quality while also enabling faster grade changes. Paper Tensile Tester
As a major supplier of laboratory and online measurement equipment - including the L&W Freeness and Fiber online system for fiber morphology measurement in the wet end - ABB has decades of experience supporting the evolving needs of the industry by pioneering many measurement techniques that have helped drive quality management forward. Such expertise is evident in the “Fiber Guide” book, which discusses the impacts of different fiber types and ways of processing as well as applications of fiber analysis within the industry. Mills looking to optimize refining and improve pulp quality can get a good basis of fiber analysis knowledge from the book. Here we'll look at ways to improve and apply new fiber morphology analysis methods for tighter control.
The original pulp freeness measurement, as measured by either the Canadian Standard Freeness test (CSF) or the Schopper-Riegler test (SR), is to a great extent a measure of the fibrillation and fines content of pulp, and simulates the drainage rate on the wet end of a paper machine. The CSF test starts with one liter of pulp suspension at a consistency of 0.3% being poured into a drainage chamber. The bottom lid and the air valve on the drainage chamber are opened to start the drainage process. The flow of water through the screen at the bottom decreases as a fiber pad builds up in the bottom of the chamber. The total volume in milliliters of water that drains from the chamber is the freeness value of the sample.
In a manually operated freeness test, variation in how the sample is handled each time a test is made means the precision and reproducibility are not as high as in automatic tests. Another drawback of manual freeness tests is the time needed to collect samples and determine pulp consistency. It is quite common that only a few measurements per sample point can be carried out in a day, and this low frequency of freeness measurements is not suitable for controlling the process with any precision. These drawbacks are easily overcome using an online automatic freeness testing system, where the samples are handled the same way every time, both in the preparation stage and in all the tests, resulting in better accuracy and precision. Plus, automatic freeness and fiber analyzers can collect samples from multiple locations and process hundreds of measurements per day—giving much more data that can be used for tighter process control.
Freeness measurements are also used to determine whether pulp has been sufficiently developed by refining. The principal objectives of pulp refining are to:
This further promotes the formation of hydrogen bonds and increases the total surface area available for bonding. When more strength is desired for a certain paper grade, the specific energy of the refining process can be increased by narrowing the gap through which the fibers are forced in the refiner. Because this results in a lower freeness, the freeness measurement is often used as an indicator of the degree of refining.
For a limited time, all mill employees are invited to download your free eBook providing a comprehensive guide for fiber analysis and process applications
Before the fibers reach the paper machine, it is vital that refining generates enough fibrillation and fine material to create proper bonding surfaces between the fibers as they are dried in the process. Too much refining generates extra energy costs (electricity to refiners, drives and vacuum pumps) and slows down the production speed (lost profit). Too little refining generates lower strength paper that could lead to web breaks at the paper machine or the converting plant. The strength may need to be increased by other means such as using chemical additives or virgin kraft as a reinforcing pulp, which are more expensive than the cost of refining. Thus, the use of online freeness to control refining can lead to reductions in production costs, web breaks, culled paper and customer claims.
An example of such use can be found at a Chilean paper mill whose ultimate goal was to control the amount of refining of the incoming pulp to meet strength specifications. Located on the heavily forested Pacific cost near Concepción, FPC Papeles makes packaging grades of paper from a combination of post-consumer and pre-consumer recovered fiber acquired from 27 cities and five customers. They wanted to improve their ability to adapt their process for the variability of incoming fibers, to both stabilize product quality and reduce costs.
An example of such use can be found at a Chilean paper mill whose ultimate goal was to control the amount of refining of the incoming pulp to meet strength specifications. Located on the heavily forested Pacific cost near Concepción, FPC Papeles makes packaging grades of paper from a combination of post-consumer and pre-consumer recovered fiber acquired from 27 cities and five customers. They wanted to improve their ability to adapt their process for the variability of incoming fibers, to both stabilize product quality and reduce costs.
For a limited time, all mill employees are invited to download the ABB Fiber Guide, which provides a comprehensive overview and deep dive of fiber analysis and process applications
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