Optimised Retention: The Key to Process, Quality and Productivity Improvements

摘要

Retention is one of the most important parameters in paper making. It tells how much of the valuable total solid materials (fibres, fines and fillers) in the pulp suspension stay on the paper machine wire and form the paper web. The mechanical retention of the paper machine wire is supported by chemical retention aids in most applications. Retention chemicals form flocks out of the fibres and fillers which are kept back better from the wire and with this increase the retention. By forming flocks the retention chemicals also improve the drainage at the wire which allows higher machine speeds. For economical reasons the retention should be as high as possible. One of the limitations, especially in graphical paper production is formation. The best formation is often achieved by lowering the retention. This means that the optimum retention has to be determined in each case. The optimum chemical retention aid can be found in lab trials. Modern lab retention and drainage measurement devices allow the shear forces to simulate at the paper machine as well as the reaction time of the retention chemicals. This leads to results that can be very easily transformed to the paper machine. Practical results are shown in this paper. For homogeneous paper quality it is absolutely necessary to have a stable retention. Stable retention can only be achieved by automatic retention aid control, not by fixed retention aid dosing levels. Retention aid automatic control, based on white water consistency, is well known. In many cases varying levels of anionic trash leads to high variations in retention aid dosage, because the mostly cationic chemical is consumed by the anionic trash. This is the reason modern control strategies combine charge and retention control to reach the goal of maximum paper quality with lowest cost. This paper shows examples of reached process, quality and productivity improvements of different paper and board mills achieved by closed loop chemical control with most modern on-line measurement technology.