Deposition Synergy Between Mechanical and Deinked Pulps, 2002 Technology Summit Proceedings

Lawrence H. Allen

Installation of a deinking plant in an integrated pulp and paper mill that uses virgin mechanical pulp (MP) often results in a greatly increased rate of pitch deposition on paper machines. As a consequence, mill operators often speak of a deposition synergy between MP and deinked pulp (DIP). This synergy has also been observed in the laboratory with a pitch deposition apparatus. The special circumstances that can lead to pitch deposition, as observation of mills using virgin MP has shown, are described. These include the introduction of the following factors into the mill: 1) high pine, 2) very fresh wood, 3) hard water soaps, 4) oil, 5) aluminium soaps, 6) white pitch (latexes from coated broke), 7) silicates, 8) fillers, 9) size, 10) microbial growth, 11) low temperature, 12) synthetic press rolls, and 13) system closure. The introduction of DIP onto a paper machine using virgin MP can bring to the furnish many of the same contaminants that are already recognized as promoters of pitch deposition. These include items 3-10 of the preceding list. It is, therefore, not surprising that poorly washed or contaminated DIP can cause pitch problems. This, by itself, explains the synergy. In addition, other contaminants can be introduced with DIP and are capable of increasing the rate of deposit growth, often by becoming embedded in sticky deposits. A new conceptual model, based in part on colloidal hydrodynamics, has been advanced for understanding pitch deposition from fundamental principles.