Approach and white water Systems - Process info
Tissue - Introduction and background
The approach system consists of the fan pump, screens headbox and wire section. To the suction side of the variable speed fan pump will be fed pulp from the machine chest, white water from the wire section flume, recirculated pulp from the headbox and accept from the secondary screen. The pressure side of the fan pump is connected to the primary machine screen and accept from this screen will be fed to the headbox. The consistency in the headbox is normally in the range 0,2-0,3%.
The screening system shown is a two stage system where the reject from the secondary screen will be taken to the white water system (WW2). The design of the screening system and location in the process is depending on local conditions (for example type of pulp and pulp cleanliness) and can differ between paper machines. Sometimes a cleaner system may also be required. Later installations also include a white water dilution system as shown in this example. The dilution system which contains a variable speed pump and a screen will take white water from the flume and introduce it to the headbox inlet. The primary purpose of the dilution system is to improve basis weight control and fibre orientation.
White water system
Excess white water from different locations in the process will be collected in a tank most often called WW2 (white water 2). (The flume is sometimes called WW1). Before the white water can be reused in the process its fibre content has to be reduced. The fibre removal can be done either with a disc filter or in a flotation process. A disc filter will be used if the white water is low on ash. The recovered fibre can be returned to the process. When the process is based on deinked pulp the white water will contain some ash (beside what is natural in the wood). In a flotation process most of the ash will be removed together with the fibre and can be disposed of to the sludge handling system. In this example where the main component in the furnish is deinked pulp the flotation process will be used.
The process is often called micro flotation or DAF (dissolved air flotation). The flotation takes place in a rectangular or cylindrical basin where contaminants and fibres are brought to the surface by help of air bubbles and chemicals. The formed surface sludge will be removed by scrapers and dumped in the recovered fibre tank. Clear white water will be taken out from the bottom of the flotation basin. The air bubbles for the flotation are created in a special system.
Clean white water is taken from the flotation basin and pumped to a pressurized vessel and saturated with air from the mill air system. The saturated water will be fed to the flotation basin and when the pressure is released air bubbles will be formed.
The rising air bubbles with the aid of chemicals will bring contaminants and fibres to the surface in the form of sludge. Clear white water from the flotation process will be fed to the CWW tank (clear white water).
Water from this tank will be used for pulper dilution, consistency control and different showers. Several pumps are located at the CWW tank due to different pressure requirements. In some shower positions an almost fibre white water will be required to prevent plugging of shower nozzles.
To fulfil that requirement some clear white water will pass a mechanical filter. This will produce a so called super clear white water (SCWW). Depending on the incoming flow of pulp to the paper machine system in combination with some addition of fresh water their will be a surplus of white water in the white water system. This excess white water can be taken to the pulping line or be disposed of to the effluent treatment plant whatever applicable.