Roll Presses

At the prevailing design, roll presses have two counter-rotating rugged rolls

installed in a machine of symmetrical layout.

Metso TwinRoll presses (Fig. 5.32) are designed for either for low (3–5%) or medium

(6–10%) feed consistency. The pulp slurry enters the press and a mat is

formed in the dewatering zone. There, the pulp consistency is raised to 8–12% as

the stock moves on between the rotating roll and the fixed baffle and the liquor

passes through holes into the inside of the roll. The clearance between the roll

and baffle can be set in order to maintain an ideal flow pattern in the press. As

the pulp leaves the dewatering zone and enters the displacement zone, wash

liquor is added to the mat;this displaces the liquor from the pulp into the inside

of the roll. In the roll nip, the pulp is pressed to a discharge consistency of 30–

35%. Finally, the pulp is discharged from the nip to a shredder conveyor mounted

above the rolls. The filtrate leaves the rolls through openings at the ends of the

rolls [39,40].

Fig. 5.32 The Metso TwinRoll press [40].

The Kvaerner Pulping Compact Press follows a similar operating principle. The

major differences are the feed technology with inlet feeding screws for uniform

distribution of pulp over the length of the press and the use of roughly three-quarters

of the drum for dewatering and washing (Fig. 5.33).

Wash press E factors are around 1, and E10 factors range between 3 and 5. The

enclosed press design generally allows high feed temperatures and minimizes

foaming and emissions to air.

The biggest advantage of press washing, however, is the high outlet consistency.

First, the amount of wash liquor per ton of pulp required for press washing is

only about half the amount needed for a washer with medium consistency outlet.

Second, the high inlet consistency to the subsequent process stage also reduces

the quantity of effluent from this stage. Third, 1 ton of pulp discharged at 33%

556 5 Pulp Washing

consistency carries only 2 tons of accompanying liquor (compared to more than

7 tons if discharged at 12%), which makes temperature and pH adjustment in the

subsequent dilution step an easier task.

As the pressing action places mechanical stress on the pulp up to a level where

liquor is squeezed out of the fiber voids, press washing can also have some benefits

in removal of extractives [42].

Fig. 5.33 The Kvaerner Pulping Compact Press [41].

5.6.6