Kraft Brownstock Washing and Filtrate Tank Level Control

Rotary vacuum washers operating at drum speeds of 1.5–3.0 rpm and black liquor solids of 15–22% generate foam in the filtrate receiver through saponified tall oil soaps and residual lignin surfactants at pH 12–13. SY-3045 is introduced into the filtrate tank or shower water at 0.05–0.20 kg/t of air-dried pulp, with the lower dosage applied to slow-speed washers and the upper bound to vacuum drum lines with high shower water recirculation. The critical process variable is not equilibrium foam height but persistence in the filtrate receiver; a sparged sample of 1 L filtrate at 0.4 L/min air is expected to show collapse below 8 s, and values above 20 s indicate either an incompatible nonionic emulsifier package or short-circuiting of the washer shower line. Overfeed above 0.25 kg/t air-dried pulp transfers unspent fatty alcohol into weak black liquor and raises chemical oxygen demand; in the first-effect evaporator this can reduce the overall heat transfer coefficient from roughly 1.2 kW/(m²·K) to below 0.8 kW/(m²·K) because of hydrophobic film deposition on 304L stainless steel tube surfaces. For linerboard and sack paper produced from this stock, food-contact compliance is evaluated under FDA 21 CFR 176.170 and FDA 21 CFR 176.180; n-heptane extractives on the finished sheet are controlled below 0.5 mg/dm², and the defoamer batch certificate is required to identify permitted C12–C22 fatty alcohols, food-grade paraffin oil, and nonionic emulsifiers with no alkylphenol ethoxylates.

What Limits Continuous Wet-End Addition on High-Speed Alkaline Fine Paper Machines?

Because modern twin-wire gap formers running at 900–1,500 m/min require a shear-stable defoamer at the fan pump suction, the wet-end dosage window is set by retention chemistry rather than by foam knockdown alone. SY-3045 requires pre-dilution to 0.5–1.0% solids with 25–35°C whitewater before the fan pump suction; direct injection into the machine chest or thick stock line creates local overconcentration and leads to clear spots in the sheet when piston metering pump stroke length is incorrectly set. The addition range for this product class in fine paper production is 0.01–0.08 wt% of dry fibre, with the lower bound suitable for softwood/hardwood blends with short circulation paths and the upper bound reserved for closed whitewater loops with conductivity above 3,000 µS/cm. A primary conflict occurs because fatty alcohol droplets can adsorb onto cationic polyacrylamide retention aid and reduce first-pass ash retention by 2–5 percentage points as measured by TAPPI T 261; simultaneous addition with anionic micropolymer or bentonite must be separated by at least 30 s of residence time to avoid charge competition. A rotating cylinder foam tester operating at 1,000 rpm with 1.0 L of whitewater provides a screening comparison: a stable product reduces dynamic foam height by 60–80% within 20 s without increasing whitewater turbidity above 50 NTU. For food-grade fine paper, migration of the defoamer into dry food simulants is checked under EU 1935/2004/EC Articles 3 and 15, and the finished sheet is specified to pass TAPPI T 437 dirt count below 5 ppm and ISO 535:2014 Cobb60 below 30 g/m².

In mixed office waste deinking systems using multi-stage flotation cells with air-to-stock ratios of 0.4–0.8 and Voith Sulzer injectors, fatty alcohol defoamers occupy a narrow operating window after secondary flotation and before thickening. Addition of 0.02–0.06 wt% on incoming recovered paper reduces carryover foam in the disc thickener filtrate without collapsing the primary froth that transports hydrophobic ink particles; the upper limit is fixed by monitoring brightness gain across the flotation stage. If the same dose is moved upstream of primary flotation, the brightness gain can fall from 12–15 ISO brightness points to below 5 points because fatty alcohol droplets spread on the froth lamellae and suppress bubble/ink aggregate transport. The product is therefore injected into the accept line of the post-flotation screening stage at a turbulence point that ensures droplet breakup to 5–20 µm using a static mixer with 6–8 elements; coalesced droplets above 40 µm create deposits on slotted pressure screens with slot widths of 0.15 mm. The resulting deinked stock, used for newsprint and light weight coated base sheets, must meet EN 646 extraction colour limits and TAPPI T 437 dirt count limits below 10 ppm after high-consistency bleaching. Compliance for food-contact use of recycled fibre is limited to non-food unless a functional barrier is demonstrated under EU 1935/2004/EC, and mills exporting to Germany are required to confirm BfR Recommendation XXXVI for the final converter.

Immediately after the high-shear mixer and filter, coating colour circulation systems on blade coaters operating at 1,200–1,800 m/min generate foam from carboxylated styrene-butadiene latex, protein or polyvinyl alcohol binders, and clay dispersants at pH 8.5–10.5. A fatty alcohol defoamer is introduced at 0.05–0.15 wt% of total coating colour solids, but only downstream of the pigment screen because high shear at the filter can strip the product from the air/liquid interface and entrain it as 10–30 µm droplets that later cause film-splitting defects. The control parameter is not air content alone but surface defect density; a drawdown bar coater applies 10 g/m² wet film weight and the dried film is inspected for fish eyes after 24 h ageing at 40°C. Dosages above 0.20 wt% lower IGT pick velocity below 1.5 m/s using ISO 3783:2019, while the acceptable release range for web-fed offset coated paper is above 2.0 m/s. For coated folding boxboard intended for aqueous and fatty food contact, compliance is assessed under BfR Recommendation XXXVI; if a polyethylene or plastic laminate is present, Regulation (EU) No 10/2011 applies and overall migration into 10% ethanol and 95% ethanol simulants must not exceed 10 mg/dm².

When Starch-Based Surface Size Circulation Turns Foam-Limited on Film Presses

Addition of an emulsified C16–C18 fatty alcohol defoamer to surface size solutions containing thermally oxidized corn starch at 4–8% solids and 55–65°C becomes necessary only when the foam layer in the run tank exceeds 10% of tank volume; lower foam layers are tolerable because fatty alcohol can accumulate on the transfer rolls and produce intermittent skip coating at speeds above 800 m/min. The dosing point is the size press service tank return line at 0.01–0.03 wt% of the starch solution; this low concentration reduces entrained air from roughly 1.2–1.8% to below 0.3% as measured by a density-based Paar loop. Higher doses cause size pick-up to fluctuate by more than 0.5 g/m² due to reduced wetting on the metering rod, and the resulting sheet shows uneven cobblestone patterns under a 20× stereomicroscope. The final sized paper, used for inkjet precoat and offset stationery, must pass Hercules Size Test TAPPI T 530 pm-89 at agent number above 2.0 s and Cobb60 below 25 g/m² according to ISO 535:2014. For European market grades, the product must be free of formaldehyde donors and must meet REACH Annex XVII restrictions on alkylphenol ethoxylates; the supplier certificate should declare APEO below 100 mg/kg and primary aromatic amines below 20 mg/kg.

Sludge Settling Deteriorates if the Defoamer Reaches the Dissolved Air Flotation Inlet

At the biological treatment stage of an integrated coated board mill, surface foam in aeration basins is generated by extracellular polymeric substances from thermophilic activated sludge operated at 35–40°C and dissolved oxygen of 1.5–2.5 mg/L. A fatty alcohol defoamer is dosed into the pre-aeration splitter box at 1–3 mg/L of wastewater flow, with the exact rate controlled by an online foam sensor positioned above the first aerobic zone. Because the product does not contain silicone, it lowers oxygen transfer efficiency by only 5–10% at 2 mg/L, as measured by respirometry with a YSI 5100 dissolved oxygen meter, whereas polydimethylsiloxane emulsions under identical airflow reduce oxygen transfer by 15–25%. Overdosage above 5 mg/L oils the floc surface and pushes sludge volume index below 80 mL/g but simultaneously raises final clarifier turbidity above 45 NTU because of dispersed pin floc. The product must be added after the dissolved air flotation unit if the mill recovers fibre in a Krofta Supracell, because addition to the DAF inlet collapses the foam that carries fines to the sludge beach. Effluent compliance for the treated water is checked against the EU Industrial Emissions Directive BAT-AELs for pulp and paper: chemical oxygen demand below 250 mg/L and total suspended solids below 35 mg/L; the defoamer itself contributes at most 20 g COD/g product, which is included in the mill’s water balance and discharge permit.