General Remarks

A wide variety of equipment is available for pulp washing, and many of these

units exist in several design variants. In this subsection we will examine some

commonly used washer types and discuss their main features. Initially, however,

some general remarks are applicable.

Clearly, every washer must provide the drainage area necessary for initial dewatering,

displacement and final thickening of the pulp. Apart from that, time must

be provided for diffusion. In this respect, the available pieces of washing equipment

show considerable differences. For example, in rotary washers and belt

washers the pulp passes through one washing stage within a few seconds,

whereas it takes minutes for the pulp to proceed through a diffusion washer.

In the case that the design of washing equipment involves the absence of a gas

phase, the possible negative consequences of air entrainment are automatically

avoided.

Washing equipment with both inlet and outlet at medium consistency does not

require intermediate dilution between washing stages. Such systems are associated

with relatively small flow rates of wash medium and extracted filtrate, thus

reducing pumping requirements and the corresponding energy consumption.

The same is true for equipment with medium-consistency feed and high-consistency

discharge. The latter has the additional advantage that the pulp washing can

be accomplished with a smaller quantity of wash liquor. Modern fiberline operations

are increasingly based on such energy-efficient and water-saving washing solutions.

It should be noted that the aim of this chapter is to provide a certain view of

industrial washing equipment. Pure dewatering equipment, such as thickeners of

all types and dewatering presses, are not discussed here, despite the fact that they

have many similarities. Any values mentioned below are typical and indicative.

Values for a specific washing application will depend upon the many factors discussed

above, and may deviate accordingly.

5.6.2