At 60–90 °C, even a thin mineral film can gut heat transfer in cement plant closed loops — 0.5 mm of mixed scale can slash efficiency ~85%. Plants are moving from phosphate programs to polymer and molybdate blends, while starting with softened or demineralized makeup water to minimize risk from the first drop.
When closed‑loop cooling in cement — think compressor oil coolers and generator bearing oil coolers — runs hot (often 60–90 °C), dissolved CO₂ escapes and carbonate/bicarbonate chemistry pushes rapid CaCO₃ precipitation (as in Eq.1). The dominant scales are calcium carbonate (rhombic/aragonite CaCO₃), calcium sulfate (gypsum/anhydrite), magnesium silicate, and calcium phosphate, all with retrograde solubility (more likely to precipitate on hot metal) (Veolia Water Technologies) (ResearchGate).
The penalties arrive fast. Kocharyan et al. quantified that a mere 0.5 mm of mixed boiler scale on exchanger surfaces could cut thermal efficiency ~85% (ResearchGate). Another review notes ~0.04 mm (1.5 mil) of CaCO₃ can reduce heat transfer ~12%, and ~3.2 mm (1/8″) correlates with ~22% loss in capacity (SlideShare) (ResearchGate). Deposits also choke flow, elevate system pressures, and drive under‑deposit corrosion.
Water scarcity and discharge limits are pushing plants toward high cycles of concentration (operating with minimal or no blowdown), which concentrates hardness, alkalinity and silica (ChemTreat). The Langelier Saturation Index, LSI (defined as pH – pHs; it combines pH, Ca²⁺, alkalinity, TDS and temperature), is the red flag: positive LSI means CaCO₃ will tend to deposit; negative means it will dissolve (Veolia Water Technologies). Maintenance teams routinely compute LSI — any positive excursion, even +0.5–1, triggers action such as more blowdown, acid feed or inhibitor adjustments (Veolia Water Technologies) (eaiwater.com).
Makeup water quality specification
A powerful first step is ultra‑clean makeup. Closed loops need tiny makeup volumes (only to replace leaks or seasonal drainage), so supplying high‑quality water — often condensate or demineralized — means “scale deposits are not a problem” in normal operation (Veolia Water Technologies). Where condensate isn’t available, zeolite softening on the makeup is explicitly recommended to preclude gradual scale buildup (Veolia Water Technologies).
Local guidance in Indonesia echoes this: keep precipitation‑forming ions low. One cooling‑tower spec advises total hardness <500 mg/L (as CaCO₃) to avoid scale and mandates softening if hardness exceeds this level (digisavior.com). Silica is similarly constrained; above 150 mg/L “silica will form very hard scale which is difficult to remove” (digisavior.com). Recommended operating windows also include alkalinity at 100–300 mg/L (as CaCO₃) and pH 6.5–9.0 (digisavior.com) (digisavior.com).
In practice, many cement plants circulate softened or demineralized water — often from reverse osmosis or mixed‑bed filters — specifically to achieve that spec. Soft makeup pushes safe cycles higher: moving from hard to softened makeup can cut Ca²⁺ input to near zero, shifting control to inhibitors rather than raw hardness. Teams commonly install a softener on the cooling feed, and where further polishing is required, a demineralizer or a mixed‑bed unit supports very low TDS and silica. Reverse‑osmosis trains are also used to generate low‑hardness makeup (Veolia Water Technologies), which aligns with deploying a brackish‑water RO front‑end.
The operational payoff is broader: with high‑quality water, closed systems are “less susceptible to biological fouling” and, with proper treatment, can “virtually eliminate…accumulation of corrosion products,” because oxygen ingress is minimal (Veolia Water Technologies).
Phosphate‑based inhibitor programs
The legacy playbook: polyphosphate or orthophosphate (e.g., sodium tripolyphosphate or hexametaphosphate) plus zinc. These chemistries sequester Ca²⁺ and form protective zinc/phosphate films on metal, offering dual scale and corrosion control, typically above pH ~7.5 (Power Engineering) (SlideShare). Orthophosphate formed from polyphosphate also coats surfaces, inhibiting corrosion.
But drawbacks are well‑known. Polyphosphate undergoes reversion (hydrolysis) to orthophosphate, which combines with Ca²⁺ to form insoluble calcium phosphate, Ca₃(PO₄)₂, on hot surfaces (SlideShare). Buecker reports that early sodium‑phosphate programs controlled alkalinity, “but problems from calcium phosphate deposition became nearly as severe as those of corrosion” (Power Engineering). Practically, operation must be at very low LSI or acidic conditions to avoid Ca‑phosphate crystallization; any slip can cause sudden fouling. Zinc addition can form zinc phosphate (Zn₃(PO₄)₂) if not carefully managed, creating sludge and under‑deposit corrosion that then requires filtration or blowdown cleaning.
Discharge rules add pressure. Phosphorus is a limiting nutrient for algae, so many jurisdictions restrict it; in Indonesia, cooling‑tower blowdown phosphate is limited to ≤10 mg/L PO₄‑P (Global Regulation). A high‑phosphate inhibitor risks non‑compliance. Buecker also documents a site where a polyphosphate/Zn program fueled algae blooms in the blowdown pond; switching to a non‑P polymer cleared the pond (photos show murky green turning clear) (Power Engineering). Zinc in effluent is similarly constrained (often ≤1 mg/L), discouraging zinc‑phosphate chemistry. Overall, these programs can work but require tight control of pH and dose — operating “on the razor’s edge” between corrosion and scale (Power Engineering). Numerous monographs caution that polyphosphates “must be applied cautiously” because of reversion (SlideShare).
Polymer and molybdate alternatives
Modern control leans on threshold inhibitors: organic polymers and phosphonates that work at very low dose (<20–50 mg/L), often paired with film‑forming corrosion inhibitors. Acrylate/maleate polycarboxylates bind Ca/Mg and adsorb on incipient crystals, modifying crystalline structure so precipitates remain small and non‑adherent (Power Engineering) (Veolia Water Technologies). They impart charge and disperse mineral fines, keeping deposits from binding even at mildly positive LSI (to ~+2 or higher) — crystals stay suspended or become soft sludge instead of hard scale (SlideShare) (Veolia Water Technologies). Plants deploy these via dedicated scale inhibitor programs.
Performance headroom also rises: co‑ and terpolymers (with sulfonate and phosphate/phosphonate groups) have been run at cycled LSI of ~3.0–3.5 without scale, enabling lower blowdown (SlideShare). In lab data from a power condenser context, 90 mg/L of a proprietary polymer (AS‑582) achieved ~91.4% CaCO₃ inhibition (MDPI); a blend of PMSA, PASP and PESA at 4:1:1 reached 91.8% inhibition while cutting phosphorus content by ~1/3 (MDPI) (MDPI).
Non‑P programs also sidestep discharge issues: no phosphorus to fuel algae, and many polymers are neutral or anionic and do not bioaccumulate (some are biodegradable). Plants have documented immediate blowdown pond improvements when phosphorus is eliminated (Power Engineering). For corrosion, non‑P programs rely on film‑formers (e.g., polyhydroxy carboxylates) or low‑toxicity metals such as molybdate, forming thin protective layers to replace older zinc or chromate films (Power Engineering), a common practice within broader closed‑loop chemical programs.
In closed loops, molybdate is primarily a corrosion inhibitor, not a scale inhibitor. Sodium molybdate is an anodic inhibitor that builds a stable iron‑molybdate film on steel (MDPI). Reference guidance recommends ~200–300 mg/L molybdate (as MoO₄²⁻) for mixed‑metallurgy coolers, with a caution to avoid very hard water (Ca²⁺ >500 mg/L) where molybdate can precipitate (Veolia Water Technologies). It is typically paired with a polymer or phosphonate scale inhibitor; blended molybdate + phosphonate formulations address both functions but molybdate’s role remains film‑forming (OmanChem). Plants treat this as a dedicated corrosion inhibitor duty alongside scale control.
Efficacy, compliance, and cost comparison
Scale control: high‑performance polymers routinely deliver >90% inhibition at practical doses — Gu et al. report 91.4% with AS‑582 and 91.8% with a PMSA:PASP:PESA 4:1:1 blend (with ~1/3 lower P) (MDPI) (MDPI). Polyphosphate systems target ≥80–90% when tightly run, but polymer programs allow operation at LSI ~+2–3 (which would deposit sludge under phosphate programs) (SlideShare). Polymers also create fewer “dead zones” because they keep crystals suspended rather than precipitated on outlets (Power Engineering) (Veolia Water Technologies).
Regulation: discharge limits are tightening. In one Indonesian region, blowdown phosphate ≤10 mg/L and dissolved zinc ≤1 mg/L apply (Global Regulation). A plant injecting 20–50 mg/L PO₄³⁻ would need substantial dilution or effluent treatment. Polymer inhibitors contain no phosphorus or heavy metals, so discharge typically needs only pH and TSS compliance. This enables higher cycles (saving water) and is a driver for NP (no‑phosphate) adoption (Power Engineering) (Power Engineering).
Economics: polymers/phosphonates cost more per kg than simple phosphate salts, but run at far lower dose. Higher allowable cycles reduce blowdown and makeup costs, while indirect savings are large — fractions of a millimeter of scale carry double‑digit heat‑transfer penalties (ResearchGate). Many operators find that modern polymer or non‑chromate programs, despite higher unit cost, reduce total cost of ownership by extending run length and cutting unplanned cleaning. Chemical delivery is typically automated via a dosing pump for tighter control.
Preventing scale from the start
Before chemistry, fix the water. Using condensate or softened makeup can virtually eliminate scale risk in closed loops (Veolia Water Technologies). Veolia warns that in “high‑Johnson Gas‑Engine cooling,” the addition of even small amounts of hard makeup causes gradual scale buildup and recommends zeolite softening (Veolia Water Technologies). (57) Thus, a proven best practice is installing softening or reverse osmosis on the cooling feed — often with ion exchange media like a cation/anion system upstream, and an RO such as a BWRO train for low hardness.
Discipline matters: regenerate softener resins on time and monitor demineralizer leakage; lock out or alarm any bypass valves that could admit raw water. Lab‑test the makeup routinely — targets include Ca/Mg ~0 mg/L, silica <10 mg/L, and very low conductivity — and configure automation so the inhibitor only feeds within spec; otherwise it should alarm. The data pattern is clear: “fix the water first” delivers longer trouble‑free runs than relying on higher chemical dosing.
Troubleshooting scale in service
Monitor parameters. Track pH, conductivity (TDS), alkalinity, Ca hardness, and inhibitor residual, and compute LSI regularly. Any positive LSI should trigger investigation; reduce cycles or raise inhibitor dose as required. Watch differential pressures and performance: a gradual drop in heat‑exchanger approach temperature or a rising inlet/outlet temperature spread indicates fouling. Chemical control is often stabilized by dedicated closed‑loop formulations dosed via a chemical pump.
Identify the deposit. If capacity drops, isolate and open the bundle. Carbonate scale is typically white, hard, crystalline; phosphate is often off‑white/brown; silicate can be gelatinous or whitish and rubbery. A simple field test: touch with dilute HCl — vigorous fizzing points to CaCO₃. If a sample doesn’t dissolve in 10% HCl, suspect silica. Confirm via lab (e.g., Ca/Al titration, XRD mineralogy).
Stabilize operations. Lower cycles‑of‑concentration (increase bleed‑off) until LSI is safely negative, then re‑optimize. If hardness is responsible, deepen softening or switch to RO product water. If alkalinity/pH drifts high, add acid feed. Fix any errant bypass or raw‑water blending.
Verify treatment. Check the scale‑inhibitor pump and controller function; verify residual by test kit or HPLC matches target (often ~5–20 mg/L polyacrylate or phosphonate). If a phosphate program is driving high phosphate levels, consider switching to a polymer or phosphonate‑based treatment; conversely, if a polymer program is fouling (rare), a small phosphate dose to sequester Ca can be tried as a last resort. Where molybdate is used for corrosion control, keep it paired with an appropriate scale inhibitor and a compatible corrosion inhibitor.
Clean and prevent recurrence. Remove deposits mechanically or chemically: carbonate scale responds to acid flushing (typically 1–5% HCl with a corrosion inhibitor) until dissolved; silica scale may require an alkaline cleaner or a dedicated silicate solvent. Neutralize and dispose per regulations. After cleaning, restart at conservative cycles, ramp inhibitor to target, and verify via lab (e.g., calcium out vs. in). Document the root cause and corrective actions, train operators on early warnings, and schedule periodic inspections.
Each plant’s details differ, but the pattern is consistent: measure plumbing and chemistry → diagnose scale vs. corrosion → correct the root cause → clean and verify. Over time, even a few tens of mg/L of hardness in makeup accumulates to many kilograms of CaCO₃ in exchangers; spending on high‑quality makeup and a well‑designed inhibitor program largely avoids those losses.
Sources: recent industry studies, water‑treatment handbooks, and regulatory documents. Nanoscale performance data and Indonesian standards were taken from current references (MDPI) (digisavior.com) (Global Regulation); guidance on chemistry and scale impact from handbooks and peer‑reviewed analyses (ResearchGate) (Veolia Water Technologies) (Power Engineering) (Veolia Water Technologies).
