Dewatering fiber suspensions

The objective of dewatering a fiber suspension in stock preparation is to separate the solids in the suspension from the water and dissolved ingredients. The reasons for dewatering the suspension are technological and economic. They are mainly... 

The driving forces for dewatering the fiber suspension can be hydrostatic, vacuum or mechanical ... 

Preceramic paper is a multi-scale composite material that is formed in the wet state from a suspension of discrete ceramic fibers and ceramic filler powder. Major steps in the preparation include coagulation of the fiber and filler in the suspension by means of retention and flocculating agents followed by dewatering of the feedstock. Figure 1 shows the composition triangle of preceramic paper. 

Dispersing is used in secondary fiber processing. It is located at the point of water loop separation where the suspension is dewatered up to a consistency of about 25 to 35 %. Often bleaching is done in combination with dispersing. For high quality demands two dispersing steps may be applied in the process. 

Drainage is opposed by the resistance to filtration, which depends on the degree of beating, chemical treatment, and type of stock, as well as on the amount of fines and fillers present. The dry content after the wire section in most cases is about 18-20%. The water removed in the filtration process (white water) carries away fibers, fines and fillers. The percentage of solids of the suspension retained on the wire, also called retention, can be increased by the addition of retention aids. The white water is reused to dilute the thick stock in the stock approach flow system. Figure 6.42 shows the filler distribution in the z-direction (across the web thickness) for dewatering the stock to only one side and symmetrically to both sides. 

The short-wave turbulence (micro-turbulence), generated in the suspension in the headbox to maintain fiber deflocculation, dissipates rapidly. For this reason, good formation requires either the fiber web to be fixed very quickly or additional turbulence to be generated in the suspension to be dewatered. This can be achieved by means of pressure and vacuum impulses from table roUs, foils and blades. However, impulses that are too strong are harmful, for example by table rolls at machine speeds above approx. 500 m min or by foils with too high a foil angle at elevated machine speeds. In special cases, on fourdrinier wires, formation is improved by agitating the wire. A shaker vibrates the breast roll and thus the fourdrinier wire horizontally in the cross machine direction with a frequency of up to 10 Hz and an ampHtude of up to 25 mm. It is used at low machine speeds and... 

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