Does Silicone defoamer improve the performance of the slurry or increase the production risk?

In the pulping process, in order to obtain white pulp, the chips are subjected to steaming, washing and bleaching processes (Fig. 1), but in this series of fiber processing, a large amount of water is consumed, and a large amount of foam is generated, so the selection is appropriate. Defoamers are critical.

Figure 1 Wood chips, post-cooking slurry, slurry after washing, bleached slurry

For many years, the paper industry has made many efforts to reduce energy, water and chemical consumption in order to achieve sustainable development. The use of silicone defoamers is one of them. The use efficiency of silicone defoamers can greatly enhance the sustainable development indicators of pulp and paper enterprises.
Silicone is a kind of polymer material, in which polysiloxane is composed of silicon-oxygen bond as the skeleton, and the structure is shown in Fig. 2. It represents the infinite possibilities of polymers. The length of the chain, the three-dimensional structure, and the type of organic groups attached to the silicon atoms can affect the properties of the silicone.

Figure 2. Polysiloxane with organic groups

The silicone oil used in the antifoaming agent is a polymer having high hydrophobicity and extremely low surface tension. Once polydimethylsiloxane (PDMS, a type of silicone) is combined with hydrophobic particles (such as precipitated or vaporized silica), a more effective defoamer is formed. The action flow is shown in Figure 3. .

Figure 3 Defoamer action process

Silicone defoamers began to be used in the pulping process in the 1980s, solving the problem of low efficiency of traditional organic defoamers. The silicone defoamer can adapt to the pH environment and high temperature conditions, and replaced the original defoamer almost overnight. Silicone has a simple structure and is easy to use. It also lays the foundation for the development of high-end defoamers in most paper mills.
Although silicone defoamers have many innovative advantages, they also inevitably cause problems such as precipitation, residual contamination, emulsification, scaling and the like.

Precipitation risk

Because silicone is hydrophobic, it has a high affinity with any hydrophobic surface. The high pH and high temperature of the liquid can promote the condensation of the residual activity of the organosilicon compound and promote the gel formation and precipitation.
Analytical studies have shown that the formation of precipitates is caused by a variety of factors, including the chemical composition of silicone defoamers. Precipitation is a concern for all pulp mills because the precipitation during the pulping process increases the cleaning costs of the pulping line.
The risk of precipitation formation depends on the composition of the antifoaming agent, the quality of the reagents, the dosage and the method of use. Therefore, pulp and paper companies can reduce the risk of sedimentation from the following aspects:
First, the defoamer dose is appropriately reduced. The use of an effective defoamer is critical, but excessive use can cause precipitation, so the dosage of the defoamer should be controlled during the pulping process and should not be used too much.
Second, an antifoaming agent having a small amount of residual substances is selected, and it is ensured that the remaining substances of the defoaming agent cannot continue to chemically react with an alkaline environment, thereby significantly reducing the amount of precipitation.
Third, the antifoam compound is emulsified using the correct formulation to enable it to be dispersed in the foamed black liquor.

Drainage problem

In addition to reducing sedimentation, drainage capacity is also critical for defoamers. The high drainage capacity means that the pulp mill can clean the pulping black liquor with less water and quickly discharge the water, which helps to improve the drying efficiency of the slurry in the later stage. This not only benefits foam control, but also significantly improves the effectiveness of the washing and production sustainability, which is also an important reason for the selection of silicone defoamers.
Therefore, although silicone defoamers have the risk of precipitation, they can reduce the energy consumption of pulping, water and chemicals through effective use. For the paper industry, it is still a sustainable and efficient one. Defoaming technology.
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