Belt Washers

In a belt washer, the pulp is transported on a synthetic wire through a number of

countercurrent washing stages. While belt washer applications range from brownstock

washing over post-oxygen washing into the bleach plant, this type of washer

is most popular where high washing efficiency is required.

The most commonly used belt washer is the Chemi-Washer (Fig. 5.28). Pulp is

fed through the headbox at a consistency of 2.0–3.5% and then distributed evenly

on the moving wire. In the formation zone, the area between the headbox and the

first shower, pulp is dewatered from inlet consistency to displacement consistency.

Here, the pulp mat is formed on the wire while liquor flows to the suction

boxes located beneath the wire. As the wire moves on, displacement washing

occurs when the mat passes under the shower where filtrate from the succeeding

washing stage flows onto the pulp. A pressure differential between the hood and

the suction boxes pushes the liquor through the pulp mat and into the suction

boxes. At each washing stage, the displaced liquor collected in the suction boxes is

sent to a shower pump, which in turn pumps it to the preceding stage. When the

pulp reaches the end of the washer, it is released from the wire and discharged

into a shredding repulper [36].

Fig. 5.28 Chemi-Washer (longitudinal section) [36] (used by

permission of Kadant Black Clawson Inc., Mason, OH, USA).

The washing efficiency of a belt washer is optimized by controlling the vacuum

profile in the suction boxes and the speed of the wire, and by adjusting the length

of the dewatering and displacement zones. Typical outlet consistencies range

from 10% to 14%, and normal dilution factors are 1–2 m3 odt–1.

In order to maintain the differential pressure between the hood and the suction

boxes, the vapor from the suction boxes is drawn into a fan and recycled back to

5.6 Washing Equipment 551

Fig. 5.29 Chemi-Washer (cross-section) [36] (used by permission

of Kadant Black Clawson Inc., Mason, OH, USA).

the hood (Fig. 5.29). Since the complete Chemi-Washer is efficiently sealed

towards the environment, the release of odorous gases is limited to a minimum.

With up to seven washing stages in a single machine, the belt washer can provide

the most efficient washing per piece of equipment of all pulp washers. The E

factor per stage is usually between 2.0 and 2.5.

5.6.4