Clasificación por Bauer McNett

Fractionation in a Bauer-McNett Classifier
R.W. GOODING and J.A. OLSON

An analytic model of the Bauer-McNett classifier (BMC) was derived and tested. The study showed that the BMC classifies fibres mainly by length. Overlap in the length distribution of classes reflects the statistical nature of the BMC operation rather than the influence of some other fibre quality. The fundamental qualityof fibre length fractionation was similar to that found in an industrial pressure screen. Un modèle analytique du classificateur Bauer-McNett (BMC) a été dérivé et mis à l’essai. L’étude a démontré que le BMC classe les fibres surtout selon leur longueur. Le chevauchement dans la répartition des longueurs des classes reflète la nature statistique du fonctionnement du BMC plutôt que l’influenced’une autre qualité de la fibre. La qualité fondamentale du fractionnement de la longueur des fibres était similaire à celle que l’on retrouve dans un classeur sous pression industriel.

INTRODUCTION
A tree is made of fibres that differ greatly in length, width and coarseness. The differences between fibres within a tree can be as great as the differences between species. However the traditionalapproach of preparing fibres for papermaking is to pulp the wood and process the fibres as a collective – refining the whole pulp to shorten the average fibre length, for example. This approach simplifies the process design, but neglects the opportunity to exploit the inherent benefits of the individual fibre fractions. Fibre fractionation is a process that segregates a blend of pulp fibres intodifferent streams based on some physical property of the fibres, such as their length or flexibility. The fibres of a particular type can then be directed to the most appropriate process and product. A pulp producer with two pulp machines, for exR.W. Gooding* and J.A. Olson** Paprican Pointe Claire, QC, Canada H9R 3J9 * Now with: CAE Forestry Systems 4635 Patricia Ave. Montreal, QC, Canada H4B 1Z2(robertg@caescreenplates.com) ** Now with: Dept. Mechanical Engin. Univ. British Columbia 2324 Main Mall Vancouver, BC, Canada V6T 1Z4

ample, could fractionate the fibres and increase the content of long fibres on one machine to provide a high-value reinforcing pulp. Alternatively a paper producer with a multi-layer headbox could direct the shorter fibres to the surface layers to improve sheetsmoothness, while placing the longer fibres in the core to provide strength. Within the mill system, one could concentrate long, stiff fibres in the feed to the reject refiner to save energy and increase capacity, while avoiding the degradation of fibres that are already acceptable. The opportunities for fibre fractionation are clear, but the use of fractionation technology in industry remainslimited. The board industry uses fractionation to optimize the character of the different layers [1–3]. In recycled fibre production, fibre fractionation and subsequent processing have been shown to improve strength and brightness, and to reduce energy consumption [4]. However the absence of equipment which can precisely, efficiently and economically fractionate fibres has limited the widespread practiceof fractionation in mill operations. The promise of fractionation has been established, in part, through the use of laboratory fractionation equipment to separate individual pulp fractions and form paper with optimal fibre blends. The Bauer-McNett classifier (BMC) is the most widespread of the laboratory devices, and it is commonly used to assess the level of fractionation obtained with pressurescreens and other process equipment. The BMC operates by the selective passage of fibres through a screen mesh. It sepa-

rates fibres mainly on the basis of length, although the fundamental principles of operation are not well understood [5–12]. The practice of using this laboratory screen to evaluate pressure screens raises several important questions: — Do a BMC and a pressure screen...