Prediction of visual impact of deinked pulp on paper cleanliness

With increasing utilisation rate of recycled raw material in newsprint and graphic paper grades, dirt specks contamination is becoming critical. Predicting sheet contamination from pulp contamination level is not straigthforward, as only a fraction of the specks are actually visible in a thick sheet. We propose a specks extinction model to predict the visible contamination in sheets of paper made of deinked pulp. The modelling makes use of the optical properties of the sheet material, and the size distribution of the specks population. The model can be used for multi-layer sheets as well. We demonstrate that it is possible to minimise visible specks contamination through a combination of stratified forming and deinked pulp fractionation.