DEVELOPMENT OF A FRACTIONATING SCREENING METHOD FOR SC GROUNDWOOD

The aim was to find a fractionating screening method for SC groundwood that would separate as selectively as possible for reject refining bo th shives and coarse, stiff, long fibres of poor bonding ability. The starting point for this study was the new screening technique develope d by Valmet-Tampella and employing narrow-slot screening baskets and a new type of C rotor. This rotor creates an even flow through the scre ening basket, and thus differs from conventional rotor models, in whic h successive suction and pressure pulses create a reciprocating pulp f low through the basket. In this study, conventional primary and second ary screening of SC groundwood on a line equipped with hole baskets wa s compared with narrow-slot screening under the same conditions. In ho le screening, the holes in the smooth baskets were 1.2-1.6 mm in size, while in narrow-slot screening, the slot widths were 0.15-0.2 mm. The long fibre yield, freeness drop and shive removal efficiency were ess entially less dependent on the reject with narrow-slot screening than with hole screening. With narrow-slot screening, a Pulmac shive remova l of 94% was achieved at a 10% total mass reject rate, compared with 7 5% removal for hole screening at double the total mass reject rate. Th e fractionating effect of hole screening is rather small. Significantl y, the new narrow-slot screening technique directs less fines into the reject than does hole screening, and also fractionates the long fibre fraction according to bonding ability. This was seen as increases in tensile index, density, tear index and light scattering coefficient of the Bauer McNett +30 fraction in the accept pulp on comparison with t he same fraction in the feed pulp. The situation was the reverse in th e reject pulp. The effect of reject rate on bonding potential was hard ly noticeable. The fractionating effect of slot screening was also cle arly reflected in the quality figures for the accept and reject fracti ons of the whole pulp.