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Vessel picking troubles of printing papers made from tropical hardwood pulps can be easily prevented by covering vessel elements with several fibers. This paper deals with the effect of covering layer on vessel picking.Using multi-layered handsheet forming device (JPRI Multi-head) two-layered sheets consisted of a thin surface layer and a thick bottom layer were prepared. Non-vessel picking pulps such as Buna (beech) or softwood BKP were used for the surface layer. Lauan BKP which contains large vessel elements was used for the bottom layer. Vessel pick tendency was evaluated by IGT printing test.The extremely thin surface layer of 2g/m2 of Buna BKP can effectively prevent picking, where single vessel element is covered only by 4 to 7 fibers. Softwood BKP also has the same effect as Buna BKP. This means that vessel pick phenomenon is controlled by small amount of fibers covering vessels. These fibers play the roll of fixing vessels on the sheet surface and of preventing initial lifting of vessels which leads to complete destruction of vessel-to-under layer bonding.Stratified sheet formation with single hydraulic headbox appears to be the most promising technology for vessel pick prevention, because it can perfectly prevent vessel picking without limitation of pulp woods used for the mid layer of the sheet.

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Experimental study with Centri-Cleaner to separate vessel elements from wood fibers was carried out using several bleached kraft pulps produced from some tropical and other hardwoods. Vessel separation efficiencies of pulp were examined in detail. It became clear firstly that vessel elements having larger projected area and lower L/D ratio (ratio of longer side length(L) to shorter one(D) of projected vessel element) were separated more effectively into reject side of hydrocyclone. Secondly, separation efficiency varied with pulp species. It was considered that one of the main reasons was the differences of shape(L/D) and size(projected area) of vessel elements. The pulp containing many slender(high L/D ratio) vessel elements had low separation efficiency. Furthermore, the separation efficiency decreased with an increase in pulp consistency for Centri-Cleaner treatment. This is probably due to the development of fiber network in a Centri-Cleaner at high pulp consistency, which would hinder the settling of vessel elements.

Principle of separation or classification with hydrocyclone is to make use of difference in settling volocity under centrifugal force among particles. It has been shown that Centri-Cleaner 600 is elective for the separation of vessel elements from wood fibers of various BKPs. This paper deals with experimental study of settling velocity of various pulp fibers, vessel elements and model particles. The results were used to explain mechanism of separation with hydrocyclone. With the model particles of vessel element, it was comfirmed that particles with large projected area and small L/D ratio (length to width ratio) settled faster in water. And the settling velocity of model vessel element was a function of its apparent density and thickness. The settling velocity of fiber, on the other hand, was proportional only to a diameter and apparent density of fiber. Effect of pulp consistency on vessel separation was also examined in terms of space available for vessel element to fall through the network of pulp fibers. The extremely low separation efficiency of vessel elements with high L/D ratio could be explained by the space availability.