Ash measurement and control- what is accurate enough?

摘要

The use of fillers and pigments improves optical and printing properties while bringing process benefits and providing savings in raw material costs.For example,the presence of fillers improves dewatering in the wire and press sections thereby reducing energy consumption.However,if papermakers are to reap the full benefits from fillers and pigments-by understanding the problems they can generate continuous measurement and control is essential.But this presents a challenge,because of the great diversity in shape,density,surface properties,charge etc-factors which make reliable and accurate measurement difficult,and the control of wet end stability more demanding.Properties,such as solubility,abrasivity,density,particle shape,etc determine how troublesome or good fillers and pigments (as coated broke) are in a furnish.The author describes a method of overcoming these problems-the combination of accurate analysers and a new control concept which focuses on white water consistency as the key to a more stable wet end.It differs essentially from previous controls in that retention aid is used to control white water total consistency.The goal is to manage the sub processes-like consistency,ash and chemistry-which have a major influence on operation of the wet end.This management is achieved by continuous measurement and automatic control.White water stabilisation is the key to wet end stability since most of the filler,fines and colloidal material returns to the process through the white water.To prevent variations in the headbox and paper wet must first prevent them in white water.But it is impossible to measure ash consistency accurately with one signal when the process involves grade and fillter changes-rough measurements which show the direction of consistency change are not enough.It is necessary to measure filler consistency very accurately-via the standard deviation of ash variation.By defining the percentage of ash at which a web break is triggered-+10% is HB,eg.-then the target ash percentage can be set to reduce the probability of breakage,at e.g.90% plus same standard deviation.The different between rough and accurate measurement is what counts when it comes to improving runnability and end product quality and optimising chemical consumption etc.To get accurate and reliable measurements,the new system focuses on the common consistency control loops:stock ash control,break time ash control,retention content (=white water consistency control) and charge control combined with retention control.The data produced enables papermakers to keep fillers and pigments at the level required to minimise runnability problems and maximise quality properties.Results from some 40 paper mills around the world show that the system also provides highly effective filler stabilisation.