Paper Machine -deposit Control Tools & Mechanisms

M any of the paper machine runnability problems that are difficult to detect, monitor, and control are caused by hydrophobic particles. These can be, for example, binder-rich particles in coated broke, wood pitch or stickies associated with recycled fiber. The most important characteristic indicating propensity for runnability problems has proven to be the size, or the increase in size, of hydrophobic particles.1,2 The concentration of wood fibers and fillers at the wet-end of a paper machine is typically less than 1%. Hydrophobic particles move freely in diluted water loops but can agglomerate due to hydrodynamic forces. As the agglomeration of these particles proceeds in the wet-end of the paper machine, a critical size of agglomerate is reached. At this point, the larger particles may start to cause deposition on the paper machine. Uncontrolled agglomeration often leads to poor runnability and defects such as holes or spots, in the finished paper. Hydrophobic agglomerates with diameters of up to 15 micrometers or more have been observed in the paper machine’s water circuits. The agglomeration of four of these particles could make a hole in a 60 micrometer thick light-weight coated (LWC) sheet. Controlling the concentration and size of hydrophobic particles in the wet-end of a paper machine can dramatically improve the efficiency of the machine. When the particle size is kept under control, the attachment, or fixation, of these particles to the fiber surfaces is stronger. The detrimental substances can then be removed from the process with the paper web, without the risk of re-deposition. The Kemira KemFlite* concept has been developed explicitly to monitor, and ultimately mitigate, the agglomeration of these hydrophobic substances.