Lo-Solids Cooking

As indicated by the name, the Lo-Solids concept [14,15]adds the feature of

reduced dry solids concentration to continuous cooking. The main reduction of

solids is achieved by the extraction of dissolved organic substances after impregnation

and by addition of wash filtrate to the cooking zones. Compared to EMCC,

Lo-Solids pulping further improves the uniform distribution of alkali and temperature

over the cook.

A typical configuration of a Lo-Solids retrofit to a single-vessel hydraulic digester

is shown in Fig. 4.146. The first screen section, often the former upper cooking

circulation (UCC) section, is used for extraction of spent cooking chemicals and

of wood material dissolved during the impregnation step. It must be remembered

that 20–30% of the total wood substance is dissolved, and a considerable amount

of alkali is consumed during impregnation. Below the first extraction at the UCC

Steam

Pulp

Extraction

Liquor

Wash filtrate

Circulation transfer

White liquor

Extraction

Liquor

Extraction

Liquor

Fig. 4.146 Typical Lo-Solids single-vessel hydraulic digester [14,15].

388 4 Chemical Pulping Processes

screens follows a short countercurrent impregnation and heating zone down to

the second screen section, often the former lower cooking circulation screens.

White liquor and wash filtrate are added to the circulation liquor and heated to

full cooking temperature.

Subsequently, the chips move into a concurrent and a countercurrent cooking

zone separated by the second extraction, before proceeding into the extended

cooking zone (Fig. 4.147). The third extraction occurs at the fourth set of screens.

Only a part of the liquor taken from the screen is extracted, while the remainder

is made up with alkali and wash filtrate and returned to the central pipe discharge.

The pulp continues to travel down through the countercurrent extended

cooking zone and is finally diluted and discharged. Compared to the retrofit

arrangement described above, a new Lo-Solids installation would omit the fourth

set of screens and respective liquor circulation.