In kraft pulp mills operating continuous digesters followed by multiple vacuum drum washers, saponified tall oil and rosin soaps released from black liquor produce a dense foam layer across the washer drum face. MY-240 40% Active Pulp & Paper Silicone Antifoam Emulsion is introduced directly into the shower water header serving the second or third brown stock washer. The emulsion is diluted with filtered mill water to 1–5% active solids before metering. Typical dose moves within 0.05–0.30 kg/t oven-dry pulp, adjusted against carry-over of dissolved lignin and residual effective alkali. At higher soap release, particularly from softwood furnish, doses near 0.20–0.30 kg/t reduce foam without destabilizing the weak black liquor soap skimming operation. Process compatibility is maintained at washer feed temperatures between 60–85 °C and pH 11–13. The product can be applied through spray nozzles at the wash drum, through filtrate tank recirculation, or at the pressure diffuser discharge. The emulsion is shear-stable enough to pass through fan nozzles with orifice diameters of 0.5–1.0 mm under conventional shower header pressure of 2–4 bar without coalescing. Mill trials commonly begin at 0.10 kg/t and move in 0.02 kg/t steps per 8 h shift to avoid over-response.

On rotary vacuum washer lines, inadequate defoamer feed leaves a foam blanket thick enough to reduce discharge consistency by 2–4% and can force stock carry-over into the filtrate system. Conversely, feed above 0.40 kg/t may form an oily film on the filtrate surface and reduce soap recovery in the mill’s tall oil acidulation plant. Foam collapse is fastest when the emulsion is added to shower water rather than into thick stock because the shower water contacts the newly formed mat surface with minimal delay. Under black liquor solids above 15%, the dose response flattens and additional defoamer does not produce proportionally lower foam height. Finished outputs from this stage include unbleached kraft pulp, linerboard, sack kraft, and market softwood pulp. Food-grade liner and sack grades refer to 21 CFR 176.170 and 21 CFR 176.180 for paperboard intended for aqueous and fatty food contact. Mill chemical management in this zone is audited under ISO 9001 quality system and ISO 14001 environmental management procedures.

What Drives Persistent Froth Across Secondary Fiber Screening and Flotation?

Deinking plants generate a dual foam system: flotation cells rely on a controlled froth to carry flexographic and offset ink particles, while downstream pressure screens and disc filters suffer from uncontrolled froth that reduces screening efficiency and increases reject carry-over. MY-240 emulsion is added only after the primary flotation cells, normally into the screen feed chest or the accepts line of the disc filter. The metered feed is diluted to 1–3% with white water before injection. Typical addition for recovered furnish runs from 0.05–0.30 kg/t dry fiber, with common dosages between 0.10–0.25 kg/t in furnish containing high levels of sodium silicate and fatty acid soap from reclaimed packaging. Premature addition into the flotation cell under pH 8.5–9.5 and 45–60 °C risks suppressing the ink-laden froth and lowering brightness gain by 0.5–1.5 ISO points.

The application point matters more than the total dose because the silicone emulsion must break surface foam without liquefying the flotation froth. Pressure screen installations with slot widths of 0.15–0.30 mm are particularly sensitive to entrained air; air pockets in the screen cause unstable accept flow and cavitation noise. Overfeed above 0.35 kg/t can destabilize the flotation froth and contribute to fiber loss in the rejects. Finished products from this line include newsprint, uncoated mechanical paper, tissue, toweling, and recycled board. Compliance for tissue and food-contact recycled board references EU 1935/2004, BfR XXXVI, and 21 CFR 176.180 where applicable, with REACH registration maintained under EC 1907/2006.

Hydrogen peroxide brightening of mechanical and chemi-thermomechanical pulps liberates oxygen at the bleach tower discharge; residual wood resin and pitch stabilize the resulting foam and can overflow high-density storage chests. MY-240 is dosed as an aqueous dilution of 2–5% into the tower discharge pump or the washer shower water. Normal treatment lies between 0.05–0.40 kg/t oven-dry pulp, with the upper end required for high-yield softwood furnishes carrying dissolved extractives. The emulsion tolerates pH 9–11 and stock temperatures of 60–90 °C in alkaline peroxide stages without breaking onto chest walls. In chlorine dioxide bleaching with pH 2–4, the emulsion remains dispersed but dose reduction is advised because oxidized extractives produce less persistent foam. Typical equipment on these lines includes twin-wire presses, disc filters, screw presses, and medium-consistency pumps handling 10–15% stock consistency.

The defoamer is injected into the stock line ahead of the medium-consistency pump to ensure distribution through the subsequent washer shower header. On high-brightness BCTMP lines, foam carry-over in the washer hood can reduce washing efficiency and force white-water overflow to the effluent system. The product should not be mixed with cationic coagulants used for pitch control at the same injection point because localized charge reversal may deposit silicone onto washer screens. Finished products include bleached chemi-thermomechanical pulp, market CTMP, coated board, and folding boxboard. Food-contact grades require compliance with 21 CFR 176.180 and EU 1935/2004; environmental discharge limits are monitored under the mill’s ISO 14001 permit procedures.

When Headbox Froth Survives Retention Aid Optimization

On fourdrinier and gap formers running at 800–1,800 m/min, headbox froth creates pinholes, streaks, and wire skip that cannot be corrected by drainage or retention aid adjustment alone. MY-240 is fed continuously into the headbox stock line or the white water silo at 0.01–0.20 kg/t dry fiber. For closed white water systems with high levels of anionic trash and starch, the dose may be shifted to 0.15–0.20 kg/t during periods of high broke addition. The emulsion is diluted with warm paper machine water to 1–5% before injection to avoid local deposition on forming fabric. Process conditions range from pH 4.5–8.5 and stock temperatures of 45–60 °C. Addition should be separated from cationic flocculant or retention aid injection by at least 30–60 seconds of stock travel time because the hydrophobic silicone emulsion can compete with cationic polymer micelle formation.

Overdosing above 0.30 kg/t commonly appears as reduced sizing response and felt fill in the press section. On high-speed gap formers, foaming is most severe at the edge of the headbox slice where air escapes from the stock jet; the emulsion is often split-fed to two points to maintain uniform surface tension. The finished paper grades include printing and writing papers, lightweight coated base, tissue, and coated packaging. A compliance matrix for wet-end use is shown below.

Finished packaging grades intended for direct food contact use the same dosing window but require mill-specific extraction tests under EN 1186 to confirm final article compliance. The wet-end application is most effective when the emulsion is added into a turbulent low-consistency stream such as the wire pit return, where rapid mixing disperses the silicone before it reaches the headbox slice.

At the size press, cooked starch and latex-bound coating color systems entrain air during high-shear mixing and return surge, causing pinholes, skip coating, and blade streaks on lightweight coated papers. MY-240 is metered after enzyme inactivation in the starch storage tank or after binder addition in the coating color tank, typically at 0.02–0.10% based on dry starch solids for size press operations. For blade coating colors containing 40–70% solids, the addition is based on wet coating weight and usually falls between 0.01–0.05% of total wet formulation. The emulsion is not injected ahead of the jet cooker at full concentration because temperatures above 120 °C and high steam shear can destabilize the silicone droplets. Coating kitchen equipment includes high-speed dispersers, ring filters, and blade coaters with backing roll speeds above 1,000 m/min.

Entrained air in coating color increases viscosity readings on a Brookfield viscometer, leading to inconsistent coat weight and calender blackening. The defoamer is introduced through a low-shear zone such as the recirculation line of the working tank, not directly into the disperser vortex, to avoid droplet coalescence. In starch size press applications, the emulsion should be added after enzyme conversion and after viscosity adjustment, because the defoamer is not designed to function as a wetting agent or leveling aid. Finished products include coated art paper, label paper, inkjet paper, thermal base paper, and folding boxboard. Compliance for coated food-contact board references 21 CFR 176.180 and 21 CFR 176.200, while EU requirements are covered under EU 1935/2004 with migration verification according to EN 1186.

Effluent Basin Foam and Oxygen Transfer Interference

Biological treatment basins at an integrated mill receive combined effluent containing residual surfactants, defoamer carry-over, and extracellular polymeric substances. Aeration creates a stable brown foam layer that reduces oxygen transfer and can overflow onto walkways. MY-240 is metered as a 1–5% dilution into the combined effluent stream before the distribution channel or sprayed directly onto the basin surface through foam breaker nozzles. Typical dose ranges from 1–10 ppm based on incoming wastewater flow, with the lower end applied to diffused air basins and the upper end to surface-aerated lagoons with high sludge age. Dissolved oxygen and mixed liquor suspended solids are monitored during initial dosing because silicone antifoam is removed primarily by adsorption to suspended solids. The emulsion is not classified as readily biodegradable under OECD 301F and should not be discharged without biological treatment.

In activated sludge plants with membrane bioreactors, the dose should be kept at the minimum effective level because silicone film accumulation on membrane surfaces can increase transmembrane pressure. Equipment on these lines includes surface aerators, diffused air headers, secondary clarifiers, and belt filter presses. The product should not be used ahead of dissolved air flotation units where controlled foam is required for sludge thickening unless separate pilot testing confirms adequate solid capture. Finished outputs are treated process water either discharged under permit or recycled to the mill, and dewatered sludge. Regulatory compliance is managed under the mill’s ISO 14001 environmental management system, EU REACH registration, and local discharge permits tied to COD and TSS limits.