Stock Preparation and Broke system - Process Info
Tissue - Introduction and background
The tissue process shown in this document is based on a furnish consisting of deinked pulp, dissolved pulp from a direct entry line (softwood, hardwood or a mixture of these) and broke.
The process shown will produce a tissue paper consisting of one ply. More demanding tissue products will often be produced on paper machines with two or three plies. These machines will have two or three approach systems respectively.
For tissue products where the demands on bulk and softness are high the TAD (through air drying) concept is often used. The perforated TAD cylinder will be located between the yankee cylinder and the reel. The improved bulk and softness will be achieved by blowing hot air through the sheet into the TAD cylinder.
If the mill also has a converting line for the production of consumer rolls it is convenient to have a converting broke pulper (CB Pulper). In this pulper rejected rolls will be dissolved and reused in the paper machine furnish. Pulper consistency is normally 3-4%. Recovered fibre (consistency 1,5-2%) from the white water system can be pumped to the broke system or the sludge handling system.
The choice depends on the quality of the recovered fibre. If for example the ash content of the recovered fibre is high it is probably best be disposed to the sludge handling.
Broke from the machine pulper (consistency 2-4%) is a third broke component. All types of broke will be collected in a broke chest. From the broke chest the pulp will be pumped through a high density cleaner (HD cleaner) and a deflaker to the machine chest. By gravity the HD cleaner will remove heavy impurities and prevent damage to the following process equipment (deflaker, refiners). In the deflaker fibre bundles will be disintegrated when the pulp passes between one rotating and one stationary disc.
The furnish components (in this case deinked pulp, softwood/hardwood from the direct entry line (and broke) will be added to the mixing chest. After mixing the pulp will be consistency controlled and pumped through two refiners in series to the machine chest.
The purpose of the refiners is to minimize pulp freeness variations and enhance pulp strength. From the machine chest pulp will be pumped with a variable speed pump to the suction side of the fan pump in the approach system.
In the mixing chest and machine chests consistencies are normally in the range 3-4%.