The pulp and paper industry is one of the most advanced segments leading sustainability today. Not only does it help divert production to biochemicals and to bioenergy, but it also reduces demands for fresh water, chemicals, and energy by continuous improvement of paper production efficiency.
Defoamers (or antifoams), which are used to eliminate foam problems, play a crucial role for the operation of pulp mills. Silicone-based defoamers are especially efficient and demonstrate many benefits, including contributing to a mill’s sustainability targets.
Silicone[i] refers to a family of polymeric materials based on an inorganic siloxane backbone consisting of repeated Si-O units (Fig. 1). It represents infinite possibilities of polymers by variation of the chain length, tri-dimensional structure and the type of organic groups linked to the Si atom, opening the door to tremendously versatile properties.
Silicone oils used in defoamers are polymers with high hydrophobicity and very low surface tension, ideal properties to make a defoamer. Once polydimethylsiloxane (PDMS) is combined with hydrophobic particles such as precipitated or fumed silica, it forms a more effective antifoaming agent (Fig. 2).
Silicone-based defoamers were introduced in pulp washing processes in the 1980s in response to lower performing organic defoamers. Their resilience to high pH and temperature made them popular and, almost overnight, they replaced existing solutions. Those silicones, still simple in structure and performance, laid the foundation for many R&D projects to design high-end defoamers used in the majority of mills today.
However, as with any innovation, silicone antifoam created new problems and technical challenges such as the risk of deposition, carryover, emulsification, dosage, injection, robustness, scaling, etc.
IS DEPOSITION A MYTH?
Due to silicone’s hydrophobic nature, silicone defoamers have high affinity with any hydrophobic surface. The very high pH and high temperature of the liquid can promote condensation of residual reactive functions of silicone compound and contribute to gel formation that can accumulate in any part of the pulp mill.
Analytical studies show[ii][iii] that such deposits are generally associated with calcium pitch and its occurrence depends on different factors including the chemistry of the silicone antifoam formulation. Deposition is a concern for pulp mills since it requires heavy cleaning of the fiber lines.
Risk of deposition will depend on various factors such as the nature and formulation of the defoaming compound, the quality of the defoamer emulsion, dosage, and how and where the defoamer is injected in the process. Mills can reduce the risk of deposition by:
• Reducing the dosage of defoamer. The choice of a very efficient antifoam can make a difference. It is reported that deposition issues at mills often resulted from an overdosage of antifoam due to ineffective antifoam performance or inadequate dosage system.
• Choosing a defoamer to leave very low levels of residual functionalities that can further react in alkaline system. Decreasing the reactivity in alkaline environments reduces the risk of deposition significantly.
• Choosing the right formulator. A robust process and formulation is key to silicone defoamer performance: silicone antifoam compound must be emulsified to be easily dispersed in the foaming black liquor.
WHAT’S NEXT FOR SILICONE DEFOAMERS?
Drainage is the most recent requirement of silicone defoamers. This brings not only improved foam control, but significant enhancement of the effectiveness of washing operation and impact on sustainability. The drainage properties represent how easy and fast it can be to clean the black liquor from the pulp with minimal water.
Drainage and dewatering are important aspects in many processes involving removal of water from a dispersion of solid material. Better and faster removal of the water (drainage) results in cleaner solid pulp and less energy for subsequent drying (Fig. 3.)
The positive impact of silicone defoamers on sustainability and carbon balance in the pulp and paper industry was evaluated 10 years ago[iv][v]. Research showed a favorable carbon balance of 27, which means that, for every ton of CO2 emitted for its production, the use of silicone defoamer allows a saving of 27 tons of CO2.
Silicone defoamers are a sustainable, robust, reliable, efficient antifoam technology for the pulp and paper industry. They can provide excellent antifoam performance, improved drainage of water, and improved robustness in a broad range of pulp mills. They contribute to the reduction of water, energy, and chemicals consumption, as well as the recovery of valuable chemicals. Silicone-based antifoams are a key contributor to sustainability efforts of the pulp and paper industry.
i W. Noll, “Chemistry and Technology of Silicones”, Academic Press, New York, 1968.
ii Bruce Sithole and Denise Filion, “Determination Of Silicone Defoamers In Mill Pitch Deposits” – APPW 2004.
iii Internal Dow chemical analysis.
iv B. Brandt, E. Kletzer, H. Pilz, D. Hadzhiyska, P. Seizov, “Silicon-Chemistry Carbon Balance”.
v S. Chao, C. Deglas, L. Vermeire “Unique Silicones Environmental Impact In Pulp And Paper”. Johan Gullichsen Colloquium and PI’s Autumn Summit 8.11.2017.