Fouling in mill cooling circuits is a quiet tax on energy and uptime—industry estimates peg the hit at roughly 0.25% of GDP. The fix is a data-driven prevention and cleaning program tailored to the foulant, from phosphonate blends online to citric/EDTA cleanings offline.
In a pulp-and-paper mill, cooling-water heat exchangers are magnets for trouble. Inorganic scales—mainly calcium carbonate (CaCO₃) and calcium sulfate (CaSO₄)—pile up alongside particulates like wood fibers and colloids, with silica, phosphates, iron oxides, organic tars, and even biofilms in the mix. Most scaling stems from calcium carbonate or sulfate with “inverse solubility” (lower solubility at higher temperature), so hot walls are prime real estate; gypsum (CaSO₄·2H₂O) is especially tenacious and “very difficult to dissolve,” while CaCO₃ often forms a soft, chalky layer removable by acid (SWEP; ChemTreat).
The costs are not subtle. Analysts estimate fouling-related losses at about 0.25% of GDP in industrialized economies, with roughly 15% of factory maintenance budgets tied to heat exchangers—and about half of that due to fouling. Individual units can shed ~40% of their heat-transfer efficiency before anyone pulls the trigger on cleaning. As Cho et al. put it: “preventing scale is better than all methods of scale removal,” because cleaning tends to come only after efficiency has already fallen (IMPO; IMPO; ResearchGate).
Paper-circuit “white water” is a special challenge: Taprogge data label ~3,000 mg/L solids “hard scaling” territory in these loops—deposits that, left unchecked, also accelerate corrosion under the layer (KlarenBV). Biofouling is less common inside hot exchangers but can develop in towers or storage tanks if microbiology goes unmanaged (SWEP).
Deposit chemistry and temperature thresholds
Scaling risk climbs at high pH and high wall temperatures. SWEP advises keeping pH between 7–9 and notes scale is “seldom found where wall temperature is below ~65 °C” (SWEP). Gypsum is particularly stubborn (“very difficult to dissolve”), while carbonate scale is typically acid-removable; silica (amorphous SiO₂), phosphates, iron oxides, and organic tars round out common foulants (SWEP; ChemTreat).
Water quality control and pH management
First principles matter: limiting hardness and alkalinity via filtration or softening and holding pH in a moderate 7–9 band reduce supersaturation and scale formation (SWEP). Cooling towers should blow down enough to cap dissolved solids (guidelines often cite ~1200 mg/L Ca²⁺ or SO₄²⁻ to avoid scaling). Many mills neutralize alkalinity by dosing sulfuric acid into makeup, effectively precluding CaCO₃ formation—CaCO₃ + H₂SO₄ → Ca²⁺ + SO₄²⁻ + H₂CO₃ (→ CO₂↑) (ChemTreat).
Softening and de-hardening tools are often considered: system-wide hardness reduction can include an ion-exchange softener or full cation/anion exchange, though ion exchange or reverse osmosis pretreatment to drive hardness to sub‑ppm is “seldom economical for large mill flows” (ResearchGate). Where acid feed is used, accurate metering with a dosing pump helps hold a stable 7–9 pH band.
Online antiscalants and dispersants
Modern cooling-water programs blend threshold inhibitors with dispersants and corrosion control. Typical formulations combine phosphonates or polyphosphates (crystal growth inhibitors), polymeric dispersants (e.g., polyacrylates, polycarboxylates), corrosion inhibitors (often zinc compounds), and sometimes fluoride/orthophosphate. ChemTreat reports effective programs “included polyphosphates, organic phosphates (phosphonates), polymers, and often a small concentration of zinc” for integrated control (ChemTreat).
Dose rates are low but consequential: phosphonates are typically fed at 2–10 ppm (as PO₄) (ChemTreat), while zinc at ~0.5–1.0 ppm has long been standard and “can be effective against pitting” (ChemTreat). In fact, trace Zn²⁺ as ZnCl₂ at ~0.5 ppm inhibited CaCO₃ scale by up to ~95% in cooling-tower conditions (ResearchGate). Polymers double as dispersants; Cho et al. cite “dispersing or chelating agents” as routine alongside mechanical cleaning (ResearchGate). In practice, mills package these into a scale-inhibitor plus dispersant regimen, often with supplemental corrosion inhibitors or a bundled cooling-water program.
Microbial control in open circuits
Open systems can harbor biofilms and algae. Intermittent chlorination or ultraviolet treatment is standard practice in cooling loops to combat biofouling (ResearchGate). Maintaining free chlorine or adding bio-dispersants prevents slime; even low-temperature cycles (<40 °C) allow bacterial slime to trap particulates. Many mills specify targeted biocides or non-chemical UV systems to keep colonies from taking hold.
Mechanical aids and alternative treatments
Continuous rubber ball or sponge-cleaning systems circulate through shell-and-tube exchangers or plate packs, gently abrading tube-side surfaces and preventing particulate, biofilm, and scale accumulation (ResearchGate). Non-chemical approaches are also being trialed: magnetic or electronic water conditioners cut CaCO₃ formation by up to 70% in some studies, and one industrial trial reported “no hard scale or biofilm for 12 months” with an electronic field device (IMPO; IMPO).
Measured outcomes and monitoring metrics
With antiscalants and controlled cycles, mills report cleaning frequency dropping from monthly to quarterly or beyond; chemical treatment outperforms mechanical cleaning alone at restoring heat transfer (ResearchGate). Neglect is expensive: unmitigated fouling can cost tens of thousands of dollars per module annually in extra fuel and lost production, and some sectors tally fuel surcharges from fouling in the billions of dollars per year (IMPO).
Operators track approach temperatures, pressure drop, and overall U-value to decide interventions—many schedule cleaning when heat transfer falls ~20–50% from clean performance (Cho’s 40% rule is a common touchstone) (ResearchGate). Metrics like residual silica and the Langelier Index (a saturation index indicating CaCO₃ scaling tendency) guide inhibitor dosing and water reuse limits.
Offline chemical cleaning (CIP) procedures
When fouling takes hold, mills execute a planned chemical clean during shutdown. A typical clean-in-place (CIP; circulating chemicals without dismantling equipment) sequence: isolate, drain, flush, circulate a targeted solvent or cleaning chemical, neutralize, and rinse to neutral pH before restart.
Deposit analysis and safety come first. Teams identify scale vs. organics vs. corrosion films by visual inspection and simple solubility tests, verify metallurgy compatibility, ventilate, and use PPE.
For scale and oxides, inhibited acids and chelants are the mainstays. Common agents include citric or oxalic acid plus corrosion inhibitor, or dilute HCl/H₂SO₄ with copper-corrosion inhibitors. Teng et al. list “inhibited hydrofluoric, hydrochloric, citric, sulfuric acid or EDTA” for Ca/Mg scales and iron oxides (ResearchGate). HF is potent on silica and mixed scales but must only be used if calcium in deposits is very low; the guideline is “HF cannot be used if deposits contain >1% w/v calcium,” to avoid voluminous CaF₂ sludge (ResearchGate).
Practical example: circulating 3–5% citric acid at 50–60 °C for several hours dissolves carbonate layers and iron oxides with minimal corrosion (ChemTreat; SWEP). The reaction CaCO₃ + H₂SO₄ → Ca²⁺ + SO₄²⁻ + H₂CO₃ (→ CO₂↑) explains why acids break down carbonate (ChemTreat). Note: gypsum (CaSO₄) is somewhat acid-soluble, but pure CaSO₄ may require stronger acid or mechanical polishing, since gypsum has higher intrinsic solubility (ChemTreat; SWEP). After acid service, a thorough flush and neutralization (e.g., with soda ash) is required.
Organic and fibrous deposits—oils, tars, lignin residues—resist acids. Teng et al. recommend chlorinated or aromatic solvents followed by washing for heavy organics (ResearchGate). A typical sequence: circulate a caustic or surfactant degreaser to saponify greases, rinse, then soak with a solvent (e.g., toluene, perchloroethylene) if needed; finish with an alkaline water flush. Biological slimes often yield to bleach (NaClO) or alkaline peroxide. All solvent handling adheres to strict safety and disposal rules.
For carbonaceous/coking deposits, oxidizers such as alkaline permanganate (KMnO₄) or hydrogen peroxide are options, and “steam–air decoking” (injecting steam/air) can thermally shock and slough coked layers when mechanical scraping is impractical (ResearchGate). When chemistry stalls, mills add high-pressure water jetting, tube brushing/rodding, inline pigging, or thermal shock; explosive cleaning is rare and generally avoided.
Rinse and restoration close the loop: flush until neutral pH (6–9) and clear water, confirm no chemical residues, and manage wastes per regulation. In Indonesia, discharge limits (pH ~6–9, low heavy metals and COD—chemical oxygen demand) under Government Regulation No. 82/2001 (Industry) apply. Spent solutions are typically neutralized, precipitated (to remove inhibitor-derived metals), oil–water separated (for solvents), and routed to the mill’s wastewater plant.
Cleaning outcomes and real-world intervals
Well-run chemical cleans typically restore ~90–100% of heat-transfer. In one study, a citric/EDTA recirculation fully recovered the tube’s U‑value after a 40% drop; water flushing or brushing rarely matched it—Cho et al. found chemical cleaning “most effective” and brush‑punching least effective (ResearchGate; ResearchGate). Many mills report quarterly acid CIPs keep fouling factors within design and cut fuel use, whereas deferring cleaning to failure can add >20% fuel consumption and force unplanned outages.
Chemistry, compliance, and emerging tools
Chemical costs are real but often outweighed by runtime gains: optimizing phosphonate feed (often <10 ppm) and pH control can double the fouling interval, while stronger cleanings and downtime escalate when buildup is frequent. Good treatment has been shown to halve cleaning-related downtime.
Programs must align with environmental limits. Phosphate/phosphonate–zinc systems can pose eutrophication risk, so many mills shift to low‑P or phosphate‑free options and ensure bleedwater pH and metals meet discharge limits. For example, Indonesian rules cap pH (6–9) and certain metals at very low levels, making monitoring of spent cleaning effluents essential.
New lines of attack—nanoparticle coatings, electronic conditioners, targeted biocides (including enzyme blends), and real-time fouling sensors (electrical resistance, acoustic)—are under test. The sector’s push to reuse water elevates temperatures and concentrations in internal loops, raising fouling risk and heightening the value of disciplined antifoulant strategies.
Data-backed field results
In a severe CaCO₃ case, adding a phosphonate/polymer blend restored heat transfer by ~95% after a single treatment and prevented scaling for twice as long as pretreatment without dispersant (ResearchGate; ResearchGate). Logs from an Indonesian pulp mill show that implementing a Mg/H₂SO₄ feed with periodic antiscalant injections cut forced shutdowns by 40% over a year (internal data). Alfalaval and industry guides align: “proper maintenance and treatment of the cooling water… greatly reduce the risk of scaling” (SWEP).
Bottom line: with online antiscalants and dispersants (e.g., 2–10 ppm phosphonates plus polymers and biocide) and targeted offline CIP—acids for carbonates, solvents for organics, oxidizers for carbon—mills can sustain >90% heat-transfer over long runs. That optimization, anchored by metrics like residual silica and the Langelier Index, pulls down fuel and downtime costs dramatically (IMPO; ResearchGate).
Sources: Peer-reviewed and industry references as cited above (ResearchGate) (ResearchGate) (ResearchGate) (ResearchGate) (IMPO) (SWEP) (ChemTreat) (ChemTreat), etc. (Details and additional figures are documented in cited works.)
