This week as we continue on in our paper making process series, we will be discussing the refining process of the pulp. In case you missed it before, the previous blogs are First Steps of Papermaking, Debarking, and Pulping Process.
In modern TMP (Thermo/mechanical pulp) plant the refining process is 2 stage or ideally 3 stage which means the pulp is fed through two or three refiners operating in series before moving forward in the process. At this point, it is important to discuss two of the most important quality control targets. They are “freeness” and fiber length. Freeness is a term which describes the dewatering properties of the pulp. The pulps ability to “dewater” has a significant effect on the pulps runnability on the paper machine. Freeness targets are largely achieved by the application of energy in the refining process and are tracked via automatic online sampling and testing equipment as well as manual sampling and lab testing. A second important quality control parameter is fiber length. Basically longer fibers yield stronger paper so the process is constantly monitored and manipulated by operations personnel to try and achieve the specific freeness and fiber length targets. As mentioned, these parameters have direct effects on the runnability of the paper machine.
Another pulping term referred to as “latency,” can be thought of as the curling of the cellulose fibers. The intense shearing forces in the refining process leave the fibers curled which is not ideal for “bonding” and strength qualities later on in the paper making process. This curling or latency is removed by pumping the now refined and diluted pulp into a latency removal tower which is a large tank where time and temperature allow the fibers to “relax” and straighten. Generally a retention time of more than an hour and a temperature of 85°C will remove the latency from the pulp. This pulp is now ready for screening and cleaning.
Screening is a critical part of the pulping process. Screening removes debris which includes undeveloped fibers or fiber bundles, referred to as shives, as well as lint, stiff undeveloped fibers, etc. all of which can lead to a weak paper sheet full or dirt and holes which is obviously undesirable. Basically a pressure screen is a large cannister inside which is an impeller power by an electric motor which forces the stock against a cylindrical screen with specifically sized slots. The “accepted pulp” passes through these slots and moves forward in the process while the “rejected pulp” which fails to pass through these slots is sent back for further refining in the reject refiner(s).
This screening of the pulp also has an effect on freeness. (Lowering it). The accepted pulp is then cleaned by various forms of cleaners, usually a centrifugal type cleaner know as a hydroclone which uses centrifugal forces to separate the pulp from impurities before moving on to the final stage of the pulp process, the bleach plant.