White water refers to the water that is removed during the formation of the paper web, as well as the water that is further removed during vacuum box and pressing. The recycling of white water refers to the use of various methods to treat paper machine white water, reduce its suspended solid content, replace clean water and reuse it in the pulp and paper making process, with the aim of reducing the amount of clean water used, reducing wastewater discharge and solid loss. The effective method in a paper mill is to use a paper machine white water circulation system to treat paper machine white water. So, what are the methods?
There are three methods for recycling white water:
1. Filtering method, such as thickener, dehydrator, Polydisc filter, etc;
2. It is an air flotation method, such as an air flotation white water recycling machine;
3. It is a sedimentation method, such as sedimentation tank, sedimentation tower, sedimentation tank, etc.
1、 Air flotation method
Due to the varying difficulty of solids floating in water, the solids in papermaking white water can be divided into three categories: first, for solids with high density that are not suitable for floating separation; Fine planktonic substances and fine flocs that are difficult to function due to buoyancy; Floating solids with high buoyancy, low relative density, or flocculating on the floating platform.
Traditional air flotation recovery equipment is only suitable for separating and recovering easily floating solids from water, while Poseidon pressurized air flotation machines can effectively remove the three types of solids mentioned above.
The traditional air flotation white water recovery device is based on saturating the white water with air, causing the solid substances in the white water to adsorb air, reduce its apparent density, and float to the liquid surface for separation.
Factors such as gas supply volume, white water concentration, and the type and dosage of flocculants can affect their recovery efficiency. The marketing of gas supply volume is relatively large, and it is necessary to control the dissolved air ratio. Excessive air volume can lead to fluid turbulence, damage flocculation, and reduce the air flotation effect; Insufficient air volume will reduce the gas-solid ratio, thereby reducing recovery efficiency.
Traditional air flotation equipment uses rectangular air flotation tanks, while the Krofta Sandfloat air flotation equipment that has emerged in recent years uses circular air flotation tanks, which have high recovery efficiency. Its operation is based on the principle of zero velocity, providing a brief residence time for white water and shallow (400mm) operating conditions.
The white water enters the purification tank through a rotating multi tube feeder, which rotates around the center in the opposite direction to the feeding direction. This reduces the movement speed of the white water to a small level, causing a relatively unknown water column to be found in the air flotation tank. The bubbles float vertically on the slurry solid to the liquid surface and are then removed with a sludge shovel.
2、 Filtration method
The filtration method is a type of method that uses various forms of filtration media to treat white water and wastewater.
At present, the representative equipment widely used is the Polydisc vacuum filter.
The Polydisc vacuum filter has compact structure, wide adaptability, high filtration efficiency, and convenient installation and maintenance, so it is widely used. It is mainly used for the concentration of various pulp and white water recovery in papermaking.
It uses the vacuum generated by the filtrate water leg tube as the filtering driving force, and the net height of the water leg is generally 5-7 meters. During operation, the liquid level in the machine slot is 100-200 millimeters higher than the centerline of the spindle.
Each sector on the disk is in different working states during operation. When the sector is brought into the liquid level, it first enters the atmospheric filtration zone. The liquid in the tank is filtered under the static pressure difference, and the turbid filtrate is discharged through the main shaft and distribution valve atmospheric filtration outlet. At this point, the sector begins to hang slurry, and then enters the vacuum zone, A large amount of filtrate passes through the suspended filter screen and is discharged through the vacuum zone filtrate outlet of the main shaft and distribution valve, forming a filter cake on the fan.
After the fan blade rotates out of the liquid level, under the vacuum suction effect, the filter cake further dehydrates, and the dryness increases to 10%~15%.
When the fan leaves the vacuum zone and enters the slurry stripping zone, the pressure water will peel off the slurry layer on the fan, fall into the slurry receiving hopper and continue to dilute with water (3% -4%), and be transported to or directly into the slurry tank using a screw conveyor.
3、 Precipitation method
The sedimentation method is the use of sedimentation to separate fibers and fillers, achieving the goal of recovering fibers and fillers and clarifying white water. The sedimentation tower is currently a commonly used sedimentation equipment, where white water enters the inner cover of the tower from the top. The function of the inner cover and the diversion pipe is to form a strong rotating fluid with white water and form a stable centrifugal sedimentation form for sedimentation.
When the concentration of white water is high, the white water will continue to settle in the cone part of the tower. At this time, it is gradually accelerated by the downward flow rate formed by the continuous discharge of tail slurry from the bottom of the tower cone. The settling speed of the concentrated white water entering the cone part forms a stable concentration gradient according to the height of the cone, which is the key to the stable operation of the sedimentation tower.
Most of the precipitated water, which has undergone preliminary sedimentation, bypasses the edge of the inner cover body, causing the awakened part of the tower body to continue sedimentation at a slow upward motion speed until the overflow ring enters the overflow groove.
At the same time, the closed circulation of white water will also have some adverse effects. White water contains various dissolved organic and inorganic substances, some of which are harmful. When the white water system is closed, these substances will greatly reduce the amount of fresh water entering the system and the amount of waste water discharged. These harmful substances will accumulate in the system, causing water quality degradation, or producing foam, or sediment and dirt, blocking mesh and blankets and other problems, Affects the wet end and paper forming performance of the paper machine.
In addition, white water sealing can also have varying degrees of impact on paper performance. According to relevant research reports, after the closed cycle of white water, the strength and tensile strength of wet paper pages are reduced to some extent, which leads to problems such as increased head breakage and reduced operational performance of the paper machine.
In addition, when the dissolved salt content exceeds a certain value, the main performance indicators of the paper page will decrease. The increase in dissolved solid content in white water also has a negative impact on the tensile strength of paper.
