Hydrocyclones

Since efficient centrifugal cleaning requires low pulp consistency and small-sized

hydrocyclones, a large number of units is required to deal with the considerable

flow rates. This has formerly resulted in long rows of cleaners with atmospheric

reject discharge. Today, the established arrangement of large numbers of pressurized

hydrocyclones is in canisters.

Figure 6.39 shows a Noss Radiclone, where the hydrocyclones are installed

radially in a pressurized cylindrical canister with vertical axis. Depending on the

cleaning capacity, one canister can hold several hundred cyclones. The feed enters

the canister centrically from the bottom and the pulp flows to the individual

hydrocyclones, where the separation takes place. The rejects and accepts from the

individual cyclones are then collected in separate compartments and leave the canister

through nozzles at the bottom. Typical cyclone diameters range from 80 to

125 mm. The pressure drop from the feed side to the accept side is between 1 and

2 bar [39]. In order to reduce the fiber loss from the last stage of cleaning, hydrocyclones

can be equipped with apex dilution.

The above-mentioned type of hydrocyclone used for the separation of heavyweight

particles is also called a forward cyclone. Cleaners for the separation of

light-weight contaminants are often termed reverse cyclones, accounting for the

6.9 Screening and Cleaning Equipment 605

inversion of accept and reject positions. Reverse cleaners can also be arranged in

canisters. The flow pattern in such canisters is similar to that illustrated in

Fig. 6.39, but the dimensions of the flow channels are adapted to the comparatively

larger apex flow rate and smaller base flow rate. In contrast to forward

cyclones, reverse cleaners do not thicken the reject, but lift the accept consistency

considerably above the feed consistency. Typical thickening factors are between

1.5 and 3.0 [40].

Larger-diameter, individual cyclones are sometimes employed for the separation

of heavy-weight contaminants to protect screen baskets or refiners from detrimental

feed components. These cyclones are typically 200–500 mm in diameter, and

may extend some meters in an axial direction. An example of a larger-diameter

cyclone separator, the Metso HC cleaner, is shown in Fig. 6.40. The cleaner can be

operated either on a continuous basis or with intermittent reject discharge as a

junk trap. HC cleaners are designed to work with feed consistencies up to 5%, but

are normally operated in the 1.5–2.5% range [41].

606 6 Pulp Screening, Cleaning, and Fractionation

Fig. 6.39 The Noss Radiclone AM [39]. Fig. 6.40 The Metso HC cleaner [41].

References 607

Дата добавления: 2015-10-21