Pulp & Paper - Paper Mill
Stock Preparation: Stock Blending
Grade change time
Pulp fibers need to be processed further before they are ready for paper making. First of all, the layers of fibers must be pulled apart to provide greater opportunities for bonding. This is done in a refiner.
The softwood and hardwood fibers, as well as any groundwood (if used), are stored separately in high density storage. There are often multiple dilution loops to get the fiber from 12-15% down to the 2-5% required for refining. Be aware that each of these loops will probably be noisy due to pulp rubbing on the electrodes.
Each furnish goes to their own refiner. Refiners are basically two discs rotating in opposite directions. Stock flow into the refiner determines the retention time. The load on the disc determines the strength of mechanical action. The temperature is measured to indicate the discs are getting too close together. If the discs clash, it is very expensive for the refiner. The ultimate measurement for these is “freeness”, and that is done in the lab.
Stock Preparation includes Hydrapulpers, which take purchased pulp bales and add water and pulp them up. Water flows can be white water, so mags are commonly used. Even if a mill has it’s own pulp mill, they typically have the ability to process purchased pulp in case the pulp mill goes down. The paper machine cannot stop for any reason! There are multiple hydrapulpers in a paper mill, as you will see. They all operate to keep the pulp stock mixed up to prevent clumping or drying and bonding of the fibers. Level and temperature are the measurements for most hydrapulpers, and flow in and out.