Overthick Wood Chip Processing in Kraft Pulping, 1997 Engineering Conference Proceedings

Since the inception of the Kraft pulping processes, the industry has understood that wood chip size plays an important role in pulping efficiency. Early chip screening systems isolated acceptable size chips from tines and oversized pieces with a multistage gyratory, vibrating, or gravity screen. The oversized chips were then rejected and conveyed to a chip crusher, and later a hammer hog or rechipper that crushes or cuts chips into smaller fractions. More recently, the industry has recognized the importance of chip thickness as well as chip size for increased pulp yield and reduced chemical consumption. Screening for chip thickness, followed by overthick chip processing requires new designs in both primary screens and resizing equipment. Disc screens, roll screens, or reciprocating screens reject overthick chips, which are now processed by either slicing or fissuring the chip. A chip slicer is a high precision rechipper that slices overthick chips lengthwise. Chip fissuring bends the overthick chips to expose more surface area. Both methods change the chip configuration to improve liquor penetration, with minimum fiber damage. This paper explores the evolution of overthick chip processing in wood chip screening systems and compares the two latest technologies. In addition, the author will attempt to quantify the costs and benefits of each.