Dispersing and HC Bleaching - Process Info
Dispersing and bleaching - introduction and background
From the screw press pulp at 30 % consistency is transported to a pre heater. In the pre heater the pulp is heated with low pressure steam to 80 – 90 °C to give optimal conditions for the following dispersing (wax and bitumen melts or softens). This is a temperature rise of approximately 35 °C.
From the pre heater the pulp falls down on the feeding screw to the disperger.
The disperger is in principal the same machine as for refining of mechanical pulp, a disc refiner.
It is almost only the pattern on the fillings that is different. The treatment occurs between two circular discs, one stationary and one rotating. In the disperger impurities as ink and wax are dispersed from the fibres. The impurities, which are divided to small particles in the dispersing, are now easier to separate in the post flotation.The dispersing may also improve the streght properties of the pulp.
Dispersed pulp is transported to a high consistency bleching tower. At bleaching the brightness will be increased to the required level. This is performed by adding hydrogen peroxide in an alcaline environment. The bleaching result depends on different variables such as hydrogen peroxide dosing, pH, temperature and consistency. The chemicals sodium silicate, caustic soda and hydrogen peroxide are mixed with water, and added between the pre heater and the disperger.
The chemicals reacts with pulp in the bleaching tower, in which the pulp stays approximately 1 hour.
From the bottom of the bleaching tower the pulp is discharged with a discharging system consisting of three or four discharging screws. Pulp falls to a screw which transports it to a tank. In the screw the pulp is diluted with white water from white water loop 2, from 30 % consistency to 7 %. In the dilution chest the pulp is further diluted, to approximately 4 %, with white water.
From the dilution chest pulp is pumped to the post flotation.