From raw‑mix chemistry to burner tuning and fuel additives, plants are using a data‑driven playbook to prevent ring formation — and, in one case, to run “without unscheduled shutdowns” for the first time in years.
For cement makers, ring formation in rotary kilns is a production killer. One modeling‑led intervention — raising secondary air around the main flame — helped a plant run “without unscheduled shutdowns” for the first time in years (link.springer.com). Another study tallied the business impact more starkly: rings were driving about one shutdown per month, each roughly three days, at a 1.3‑Mt/yr line — a hit that can run on the order of ~$100,000–$150,000 in lost production for just a single 3‑day outage, depending on plant size and clinker value (link.springer.com).
The fix isn’t one lever. It’s a coordinated approach: raw‑mix chemistry (the ratios of oxides in kiln feed), kiln operating parameters, fuel quality, and, increasingly, chemical additives — all tuned to suppress the sticky, low‑melting phases that glue dust to the shell. What follows is a field guide built from industry notes, vendor data, and case studies, with reference URLs embedded for verification.
Raw‑mix chemistry control
Ring formation is tightly linked to feed chemistry. Excess fluxes and alkalis (Fe₂O₃, Al₂O₃, MgO, and Na₂O+K₂O) lower the melting point and increase the liquid phase (the molten fraction) at clinkering temperature; “kiln feeds with a high liquid phase (easy‑burning mixes)” — rich in Fe, Al, Mg and alkalis — “are more effective in coating (and ring) formation” (cementequipment.org). By contrast, “hard‑burning” mixes (high CaO/SiO₂ ratio, low flux) yield less liquid and are less ring‑prone.
Industry guidance centers on alkali–sulfate balance. A rule‑of‑thumb is to keep the molar SO₃/(Na₂O+K₂O) ratio near unity. Sulphur‑rich mixes (S/alkali > 1.2) can generate CaSO₄ in kiln gas that sticks to particles; very low S/alkali (< 0.83) encourages low‑melting alkali salt rings (cementequipment.org) (cementequipment.org).
Practical adjustments follow from that chemistry. Plants limit Na₂O+K₂O in the kiln feed (e.g., < 0.6–0.8% by weight) and match sulfur intake from fuels; they monitor the Alkali Ratio (AR = (Na₂O+K₂O)/SiO₂) and the alkali‑sulfate ratio to avoid excess free alkali vapors. If raw or fuel streams bring high sulfur, adding Ca or Mg sources can capture SO₂ as CaSO₄ or MgSO₄ rather than letting it bind deposits. Patent literature notes MgO reacts with SO₂ to form stable MgSO₄, blunting alkali sulfate deposition (patents.google.com). Simple steps like substituting part of the limestone with dolomite or adding quicklime can tie up sulfur. Field data connect these ratios directly to ring type: “sulphur‑induced rings” at S/alk > 1.2; “alkali rings” when S/alk < 0.83 (cementequipment.org) (cementequipment.org).
Controlling the liquid fraction in the burning zone is pivotal. Modeling and plant experience suggest that ~24% liquid phase under the flame is enough for a strong coating to form (cementequipment.org), so operators aim for raw‑to‑clinker formulations that produce only about 15–20% liquid at peak temperature. Rich clays or shales (high alumina) raise the liquid phase and can be offset with more hard limestone or silica. Monitoring the Silica Modulus (CaO/SiO₂) and the Alumina Modulus ([Al₂O₃ + Fe₂O₃]/K₂O+Na₂O) helps keep melt chemistry in a safe range. Clinker targets (C₃S, C₂S, C₃A) should absorb available CaO and limit free belite (2CaO·SiO₂), since free belite can link with kiln‑gas CO₂ or SO₂ to form hard “spurrite” یا “sulphospurrite” rings (Ca₅Si₂O₈·CaSO₄) (cementequipment.org) (slideshare.net). Ensuring complete calcination and avoiding excess free CaO in the feed minimizes such carbonated rings.
Some raw‑mix tweaks can change the melt behavior. A study on SiO₂‑impregnated alumina bricks showed inhibition of sulphospurrite by stabilizing the dicalcium silicate phase (slideshare.net); the analogous logic for raw meal is to blend in finely divided silica or alumina (e.g., pozzolans, milled sand) to bind alkalis into higher‑melting silicates. Incorporating small amounts of magnesia (MgO) — for example via dolomitic limestone or magnesian clays — can increase the liquidus temperature of alkali‑bearing compounds; fuel‑side patents explicitly promote MgO in the system to scavenge SO₂ and vanadium (patents.google.com). Any such addition must respect cement limits (MgO typically < 2–3% in clinker).
Kiln operating parameters
Rings flourish where local temperature and gas dynamics generate oversized liquid fractions or sticky particles. Computational modeling (CFD, or computational fluid dynamics) shows hotspots coincide with zones of maximum radiative heat flux (link.springer.com).
Flame shape and burner tuning matter. A long, centered flame can sweep deposits downstream; overly short or wide flames can impinge and overheat locally. In one CFDemodeling case, shifting to a longer flame (by reducing tertiary air jetting) lowered peak wall temperatures and suppressed rings. Raising the secondary/primary air ratio reduced hotspots and liquid fraction peaks, and after increasing secondary air flow the plant reported running “without unscheduled shutdowns” (link.springer.com).
Kiln speed and taper also play a role. Rotation and inclination are tuned so material flows steadily without a “hole” (cold kiln) or overly slow transit (overfiring). Typical kilns run at 0.7–3 rpm (revolutions per minute); stability, not shocks, is preferred, even if sudden speed changes can dislodge a ring.
A smooth temperature profile is essential. Plants install multiple pyrometers or thermocups to spot unstable zones early. If a hotspot is detected, adjusting the fuel/O₂ ratio or tertiary air can reduce liquid‑phase spikes; one documented intervention increased the overall air‑to‑fuel ratio to distribute heat and prevent runaway melting (link.springer.com).
Dust and false air control helps keep conditions steady. Bypass air leaks cool the flame and create fluctuations that encourage rings; improving seals and tuning the preheater bypass duct stabilizes the flame. Excess fine dust recirculation (from cooler return) is reduced because fine particles can “glue” onto clinker when a liquid phase is present.
Even with vigilance, some buildup occurs. Plants schedule routine off‑line cleaning (quenching preheater, milling deposits from risers) to prevent large rings. Removing a developing ring early, before it becomes a dam, limits losses; operators typically remove rings with cutters or via controlled cold burns every few weeks.
Fuel quality and feedstock type
Fuel chemistry often sets the stage for deposits. High sulfur or chlorine fuels exacerbate rings. Coals or petcoke with S > 1–3% emit SO₂/SO₃ that form low‑melting alkali sulfates unless neutralized; chlorine from PVC, biomass, or marine fuels forms alkali chlorides. These compounds melt at ~800–900 °C, bind dust, then freeze into rings. Mitigation options include lower‑S/Cl fuels, blending high‑S coals with cleaner coals or natural gas, or raising raw %CaO or adding lime/magnesia to capture SOx. One patent recommends dosing MgO into fuel to trap SO₃ and vanadium: Mg(OH)₂ + SO₂ → MgSO₄ + H₂O (eq. 10–12 in patents.google.com).
Vanadium is another hazard. Heavy oil and some petcokes carry vanadium that forms sticky vanadates. Operators avoid sources with > 1000–2000 ppm V; if present, they mitigate with Mg or Ca additives (patents.google.com). In the cited patent, target fuels had 4.5–8% S and 500–5000 ppm V — an extreme case — yet were rendered inert to buildup by intimately mixing sub‑20 µm Mg(OH)₂ into the fuel (patents.google.com).
Ash content matters. High‑ash coals (20–30% ash) leave more particulates that can melt into glassy rings. Shabana notes that “in kilns fired with high‑ash coal, rings can form ~7–8.5 diameters from the outlet” composed largely of molten coal ash (cementequipment.org). Plants therefore favor lower‑ash coals, careful blends, and complete combustion to reduce sticky fly‑ash carryover.
Alternative fuels bring diverse chemistries: plastics/oils (high Cl, S, alkalies) and agro‑waste (e.g., silica in rice husk ash). Local rules apply (Indonesia allows certain wastes in co‑processing), and operators screen composition carefully. When a new waste stream is introduced, pilot testing deposit behavior is standard; Indonesian kilns using palm shell/bagasse have reported increased alkali cycles and coating. Blending waste with coal or adding more CaO in raw can offset these effects.
As alternative fuel use increases globally, ring issues have risen. Standards in some countries cap S, Cl, or heavy metals in fuel streams (e.g., CIWM UK guidance or Indonesia’s MEMR/Figure‑of‑Merit rules for hazardous waste in kilns). Even where regulations do not mention ring formation, these limits indirectly address the problem.
Chemical additives and inhibitors
Operators increasingly deploy chemical inhibitors to alter deposit chemistry, either on the fuel side or in the raw feed. One example: products like T&S’s Adi3tek add surfactants, polymers, and catalysts to the burner feed to “re‑direct sulfur and trace element reactions” so alkali compounds form higher‑melting, non‑sticky phases (zkg.de). In practice, operators add roughly 1 L of inhibitor per ton of solid fuel; reports claim this “inhibits ring formation and removes pre‑existing rings,” enabling continuous operation (zkg.de) (zkg.de). Plants typically meter such rates precisely; accurate chemical dosing hardware such as a dosing pump supports repeatable addition at the burner.
Patented fuel‑side strategies also dose fine MgO or Mg(OH)₂ so Mg captures SO₂ to form MgSO₄ (and binds V), preventing CaSO₄ melt and vanadate sticking (equations 10–12 in patents.google.com). One scheme handled fuels with up to 8% S and several thousand ppm V by intimately mixing sub‑20 µm Mg(OH)₂ (patents.google.com). Plants sourcing such inhibitor packages often evaluate specialty chemicals designed for deposit control.
Raw‑side additives mirror the same chemistry. Periodic addition of magnesium carbonate or dolomitic lime increases MgO to tie up SO₂/Cl and raise the liquidus of any alkali‑flux. Bauxite or iron ore fines can shift alumino‑ferrite melt behavior (within clinker spec limits). Experimentally, SiO₂ or P₂O₅ additions can raise slag viscosity. The principle: doping the melt with higher‑melting components reduces sticking. By contrast, CaF₂ or other low‑melting fluxes are known to worsen rings.
Operational adjuncts show up too. Spraying small amounts of fine raw meal dust or recycle glass into the flame can act as a transient thermal shield to reduce surface temperature spikes, though this is rare due to flame‑stability risk. More common is ensuring combustibles or binding agents (e.g., recycled oils) do not leave sticky residues.
Measured outcomes and economics
Two datapoints anchor the business case. First, increasing secondary air — with no change to raw or fuel — eliminated unscheduled shutdowns and produced “huge cost savings” in a documented case (link.springer.com). Second, a vendor reports that using their inhibitor allows ~20–30% higher sulfur feeds without rings (zkg.de). Given that one plant was losing roughly 3 days per month to rings at 1.3 Mt/yr capacity, preventing a single 3‑day outage can be worth ~$100,000–$150,000; even if additives cost thousands per shipment, they can pay for themselves quickly (link.springer.com).
The integrated playbook is clear: balance fluxes and alkalis in the raw mix; tune flame shape and air ratios to even out heat; select or treat fuels to minimize sticky ash precursors; and, where needed, apply inhibitors (fuel‑ or raw‑fed) to blunt deposition. Plants combining these measures — with ongoing monitoring — report “dramatic” reductions in ring stoppages and higher clinker throughput, including a sudden rise in on‑line days after starting an inhibitor (zkg.de) (link.springer.com).
Sources: industry and research literature including case studies and technical notes (zkg.de) (cementequipment.org) (link.springer.com) (slideshare.net) (patents.google.com). All recommendations and figures reflect the cited sources.
