Bleaching And Dewatering - Process Info
Bleaching and Dewatering - Introduction and background
From the TMP pulp storage tower the pulp is pumped at approximately 5 % consistency to a dewatering wire press prior to the high consistency bleaching tower. The wire press dewaters the pulp to 35-38 % consistency, the right consistency for bleaching. Several filtrates are taken out from different parts of the press. The filtrates are led to two or more filtrate tanks to keep cloudy and clear filtrates separated. Cloudy filtrate is used for dilution prior to the wire press and for flushing press inlet pipes. Clear filtrate is used as wire cleaning shower water.
At bleaching the brightness will be increased to the required level. This is performed by adding hydrogen peroxide in an alkaline 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 to a high consistency mixer, after the wire press. Pulp (with chemicals added) is transported to high consistency bleaching tower The chemicals reacts with pulp in the bleaching tower, in which the pulp stays approximately 2 – 3 hours.
From the bottom of the bleaching tower the pulp is discharged with a discharging system. Pulp falls to screws which transports it to a MC stand-pipe. In the last screw the pulp is diluted with white water from the paper machine, from 30 % consistency to 10 – 12 %. Bleached pulp is further diluted in the bottom of the MC stand- pipe to keep a consistency of 8 – 10 % as it is pumped to a dewatering press after bleaching.
This post-dewatering of bleached pulp works like a washing of the bleached pulp in order to reduce the amount of organic compounds and residual chemicals to be sent to the paper machine. After the press pulp is diluted in a chest and pumped to storage or directly to the paper machine.
In this section the sealing water system is not shown in detail. Only one position (OS-469) is shown as reference. There are however several other consumers as pumps, aqitators, conveyors, vacuum pump etc. They should all be arranged in the same way as OS-469.
There may also be hydraulic equipment that will require cooling water that should be arranged in the same way as shown in the section ”Refining” regarding refiner hydraulic systems.