XIAMETER ACP-3073 Chemical Pulping Heavy-Duty Silicone Antifoam is a 100 wt% active silicone compound formulated for heavy-duty defoaming in chemical pulp mills. The product combines high-molecular-weight polydimethylsiloxane with hydrophobic silica particles; the silica component functions as a foam-film puncture aid, not as a filler, allowing the low-surface-energy silicone phase to enter and rupture persistent black liquor foam lamellae. Typical physical properties in the manufacturer’s technical data sheet include an off-white to grayish liquid appearance, density of 0.98–1.02 g/cm³ at 25 °C by ASTM D4052-22, and Brookfield rotational viscosity of 2,000–4,000 mPa·s at 25 °C by ASTM D2196-20. The product is dispersible in aqueous process streams and is intended for liquor environments where pH exceeds 12, dissolved solids exceed 45 wt%, and temperature ranges from 60 °C to 95 °C. Active silicone content is controlled by lot-specific certificate of analysis. The compound may be metered as supplied through positive-displacement or high-accuracy diaphragm pumps; if dilution is required to improve distribution in long headers, process-compatible dilution at 1:1 to 1:10 with mill water or clarified white water is acceptable under gentle mixing. Compared with water-based silicone emulsions of 10–30 wt% active content, this product reduces freight, storage volume, and freeze-thaw sensitivity. Compared with mineral-oil or polyglycol defoamers, it does not saponify in strongly alkaline liquor and adds no fatty acid residues to the chemical recovery cycle.

In kraft and sulfite chemical pulping, foam is generated by surface-active tall oil soaps, residual alkali lignin, and degraded hemicelluloses. The resulting froth reduces washer displacement efficiency, entrains fiber into filtrate channels, and increases evaporator carryover. Dosing locations are selected upstream of the foam source: blow-tank discharge lines, knotters, screen feed chests, washer shower headers, and weak black liquor storage tanks. The addition rate is normally set between 5 and 50 g per ton of dry solids in the liquor stream, but mill-specific demand can exceed this band during soap skimming peaks, after chip quality changes, or when a mill closes water circuits and raises dissolved organic loading. The product is injected through quill ports downstream of the last high-shear device; injection before a pressure screen or centricleaner cascade can over-disperse the silicone into submicrometer droplets and reduce film-breaking efficiency.

Production-scale control of ACP-3073 addition is normally performed by flow-based feed-forward control from the weak liquor flow meter, with a trim signal from foam height or condenser differential pressure. Because the product is 100 wt% active, the required volumetric flow is often below 100 mL/h on smaller fines lines; this places the metering pump into the lower 10–20% of its stroke range if a larger pump is retained from an oil-based defoamer program. Positive-displacement gear pumps with variable-frequency drives, peristaltic pumps with chemical-resistant tubing, or solenoid-driven diaphragm pumps are preferred. A continuous water flush of 0.5–1.0 L/min at the injection quill prevents black liquor scale from blocking the discharge nozzle. The dilution water and product should be mixed in a low-shear mixing tee no more than 60 s before injection; longer residence times in diluted form can allow water-in-silicone separation in low-flow areas.

Why Does the Product Remain Active in High-Alkalinity Black Liquor While EO/PO Defoamers Lose Efficiency?

Black liquor foam films are stabilized by anionic soaps and elevated bulk viscosity. Conventional ethylene oxide/propylene oxide block copolymers lose surface activity when the liquor temperature exceeds their cloud point, typically between 70 °C and 85 °C; at this transition, the defoamer partitions into the bulk liquor or forms a separate oil layer and leaves the air–liquor interface unprotected. ACP-3073 does not depend on a cloud-point transition in the 60–95 °C operating band because the silicone fluid is insoluble in aqueous black liquor. The silicone droplets retain low interfacial tension, and the hydrophobic silica particles create surface defects with contact angles above 90°. When a foam lamella thins to a critical thickness below 10 μm, the particle bridges the two interfaces and dewets the liquid film, producing local rupture. This bridging-dewetting mechanism operates at film thicknesses encountered in washer hoods and evaporator vapor bodies.

Mechanical shear in centricleaners, pressure screens, and recirculating evaporator pumps can emulsify defoamer droplets and reduce particle size. Heavy-duty silicone compounds resist shear-induced break-up because their high zero-shear viscosity limits droplet deformation during residence times of 30–120 s in centrifugal cascades. In production-scale screen rooms with aperture velocities of 1.5–3.0 m/s, the dispersed droplets remain sufficiently coarse to enter foam films. If the product is over-dispersed by in-line static mixers or centrifugal pumps, the bridging mechanism becomes less efficient; the recommended installation is therefore a quill injection point after the last high-shear device and before the foam-control target. The exact droplet-size distribution can be checked by optical microscopy or laser diffraction on diluted process grab samples, but the mill must standardize sampling temperature and liquor solids to avoid artifacts.

The antifoam action of silicone/silica compounds is strongly influenced by the size ratio between the silica particle and the foam film thickness. If the silicone droplet is too small, it may remain in the Plateau border without entering the lamella; if the droplet is too large, it may break the lamella but leave oil patches on fiber surfaces. Mill application trials typically start at the low end of the dose range and increase by 10–20% increments every 4–6 h to allow the washing stage to reach a new steady state. Foam height in the screen room can be monitored by camera or differential pressure, but the most reliable process indicator is the vacuum pump seal water clarity and the filtrate tank overflow loss. The product’s ability to control foam in black liquor is not a chemical reaction; therefore no stoichiometric equivalence with soap concentration is required. The action is kinetic and depends on droplet transport, interception, and film rupture frequency.

Falling-film evaporator trains concentrating black liquor from 15–18% to 65–75% solids are particularly sensitive to foam carryover because liquor circulation rates are high and soap recovery is integrated with the liquor cycle. The product is added to the weak black liquor feed tank or before the first-effect recirculation pump at 0.5–2.0 ppm active silicone based on weak liquor mass. At solids above 55%, black liquor viscosity rises sharply; this rheology change slows lamella drainage but also increases defoamer diffusion time. In a 5-effect forced-recirculation train with a surface condenser, foam carryover is commonly indicated by a rapid increase in condenser conductivity or by foaming in the hotwell. Mills that switch from oil-based defoamers to ACP-3073 may observe a lower organic addition per ton of black liquor solids because the active dose is 10–50 times lower by mass, but the exact factor varies with soap concentration, liquor temperature, and feed location. Published data for this specific configuration is limited; the values stated here are derived from supplier application literature and production-scale control logs.

Black liquor evaporators are prone to calcium carbonate and sodium sulfate scaling; the presence of silicone antifoam does not materially alter scaling rates, but if the defoamer is overdosed, silica particles can accumulate in recirculation pump strainers and reduce the heat-transfer coefficient. Routine boil-out with weak acid or sulfamic acid should be scheduled after any antifoam product change, not because the product itself causes scale, but because the previous oil-based program may have left hydrophobic deposits on the tubes.

Specification Boundaries, Receiving Checks, and Storage Limits

Receiving inspection should compare lot density and viscosity against the certificate of analysis. A deviation greater than ±5% from the target viscosity may indicate separation or improper homogenization; the lot should be mixed with low-shear agitation before use. Storage tanks should be stainless steel or high-density polyethylene. Carbon steel is acceptable for short custody transfer but may introduce iron contamination. The product should not be heated above 60 °C during storage because prolonged heating accelerates silica dewetting and may increase oil carryover. Freeze-thaw cycling below 0 °C is not recommended; if freezing occurs, the product should be warmed to 20–25 °C and recirculated with a positive-displacement pump until homogeneous. Repeated freeze-thaw cycles can broaden the particle-size distribution and reduce batch-to-batch reproducibility. The product’s viscosity is temperature-dependent; pump sizing must use the lowest expected storage temperature, and only low-pressure steam tracing should be used if ambient temperatures remain below 5 °C. Heating above 60 °C is not recommended.

Under FDA 21 CFR 176.170, components of paper and paperboard in contact with aqueous and fatty foods are cleared provided they are used in accordance with good manufacturing practices and the finished article complies with applicable limitations. Under 21 CFR 176.180, components of paper and paperboard in contact with dry food are similarly addressed. ACP-3073 may also be listed under 21 CFR 173.340 as a defoaming agent for specific food-processing applications, but the chemical pulp mill use should be confirmed through the supplier’s regulatory affairs document for the target packaging grade. The mill’s food-contact assurance program should document the maximum use level by weight of dry fiber or surface area, the extraction test method used to verify compliance with the finished article, and the supplier lot number for traceability. The product is supplied under EU REACH registration obligations; the downstream user must confirm that its use is covered by the supplier’s exposure scenario. For equipment sold into EU markets, the product does not contain lead, cadmium, mercury, hexavalent chromium, polybrominated biphenyls, or polybrominated diphenyl ethers at concentrations above the maximum allowed by RoHS Directive 2011/65/EU.

When a Heavy-Duty Silicone Compound Replaces Oil-Based or EO/PO Defoamers in a Closed Water Circuit

Changing antifoam chemistry has consequences for secondary treatment and pulp quality. Oil-based defoamers introduce long-chain fatty acids and mineral oil that increase biological oxygen demand in the effluent; they may also saponify in kraft liquor to produce additional soap and increase foam load at the source. EO/PO block copolymers add organic carbon that is slowly biodegradable; a portion may survive the activated sludge basin and contribute to effluent COD. A 100 wt% polydimethylsiloxane defoamer is biologically inert under aerobic wastewater conditions and is retained primarily in the process water loop. The main operational difference is that silicone compounds must be added in very low quantities; overdose can create silicone deposits on sheet-machine clothing or on evaporator heat-transfer surfaces if the product is injected upstream of a low-shear separation point. Mills using felt-cleaning or process surfactants should avoid simultaneous injection of high-HLB synthetic surfactants that can emulsify the silicone into submicrometer droplets and reduce foam control. Separate injection points and sequencing are required when the machine uses polyamine wet-strength resins or cationic retention aids in the same process water.

Within the silicone antifoam category, ACP-3073 is distinguished from general-purpose defoamers by its higher high-shear stability and lower water content. General-purpose silicone compounds may contain emulsifiers or lower-viscosity silicone fluids that are optimized for ambient-temperature waste treatment, not for hot alkaline black liquor. ACP-3073 is delivered as a neat compound rather than a stabilized emulsion; this is the primary reason for its heavy-duty classification. It should not be confused with water-dilutable silicone emulsions that are supplied at 10–30 wt% active and require biocide preservation.

The substitution is not a drop-in viscosity-for-viscosity change. Feed systems must be recalibrated because low dosing rates require positive-displacement metering pumps with stroke lengths above 10% or high-accuracy diaphragm pumps; water flush lines should be sized at 0.5–2.0 L/min to keep the injection quill free of black liquor scale. The product is incompatible with strong oxidizing acids and should not be blended with solvent-based deposit-control agents or cationic retention aids in the same feed line; precipitation can occur if local pH drops below 2 or if polyamine wet-strength resins are introduced without a separate injection point. The product should be stored in sealed containers to prevent moisture ingress and surface debris that could interfere with the metering pump check valves.