Circulation Phenomena In The Clinkerization Process: Complet

Circulation phenomena are the invisible chemistry of the rotary kiln system. Potassium, sodium, sulfur and chlorine enter with the raw meal and the fuel, partly volatilize in the burning zone, condense in the cooler parts of the system, and ride back into the kiln with the meal and the dust. This closed loop of evaporation and condensation concentrates these minor components far above the levels of the raw feed: the internal circulation can multiply the effective chloride concentration by ten to a hundred times, and it is the root cause of preheater blockages, coating rings at the kiln inlet, snowmen in the cooler, filter corrosion, and the unstable burning that costs plants days of production every year.

The Complete Cement Technical Package (931 files: the kiln process courses, the chemistry handbooks, the Excel balance tools and the operating manuals: $249.99 one-time purchase, instant download and lifetime access through the PayPal payment link) includes this circulation file with its volatility tables, cycle calculations, bypass design data and troubleshooting registers. This article explains the phenomena the way the file teaches them: the chemistry of each volatile, the types of cycles, the limiting concentrations, the bypass, and the operational measures available before any capital is spent.

The one idea that holds the whole subject together: the circulating components leave the kiln system through three doors only. They leave with the clinker, with the exported dust, or with the gas through the bypass and the stacks. Every tonne that enters but cannot leave through one of those doors accumulates in the system, and the accumulation is exactly what the operator sees as coating, ring and blockage. The whole discipline of the process engineer is, in the end, bookkeeping: make the volatile balance close, and the kiln runs.

1. The Three Volatile Families: Sources and Destinations

The behavior of a compound in the kiln system is decided by one laboratory number: its volatility at the burning-zone temperatures and the temperature at which it condenses again. The ranking from the file’s tables:

• Chlorides (Cl): the most volatile and the most dangerous family. The potassium and sodium chlorides volatilize almost completely in the burning zone and condense below about 800°C, which is where the upper preheater cyclones live. The chloride cycle is nearly total: 90–99% of the chloride entering the kiln does not leave with the clinker, it circulates until a bypass or a dust export removes it. The practical limit for stable running in a preheater kiln without a bypass is about 0.015–0.025% chloride in the raw meal; above the limit, the preheater cyclones clog on a schedule of weeks, not months.
• The alkalis (K2O, Na2O): potassium is more volatile than sodium. The alkalis volatilize in the burning zone as elemental or sulfate forms and condense in the preheater, and 70–90% of the alkali input circulates. The exit doors for the alkalis are the clinker phases (the sulfoaluminates) and the dust; the useful raw-meal limit without a bypass is around 1.0–1.5% equivalent Na2O, but the real answer is always the sulfur-to-alkali ratio, because the two families travel together.
• The sulfur family (S as SO3): sulfur enters as pyrite and sulfides of the raw material, as organic sulfur and as fuel sulfur. The volatile sulfate forms condense in the 800–1000°C zone of the lower preheater; the non-volatile part leaves with the clinker as calcium sulfate. The sulfur cycle factor is typically 1.5–4, far below the chloride factor of 10–40, but the sulfur matters out of proportion to its own factor because the alkali circulation almost always travels as the sulfate.

The exit routes deserve the same clarity. The alkalis and the sulfur leave with the clinker in the thermodynamically stable phases, with the fine dust (the cyclone, the filter and the bypass dust), and with the gas. The volatile balance of the plant, computed monthly from the feed, fuel and clinker analyses, is the statement the process engineer attaches to every operating report, and its unexplained fraction is the measure of the deposits that have accumulated inside the system.

2. The Three Cycles: Internal, External and the Dust Return

The file makes a professional distinction between three separate recirculation paths, and the confusion of the three is the engine of many wrong diagnoses:

• The internal cycle (kiln system): the component volatilizes in the burning zone, the gas carries it up the preheater, it condenses on the meal and the dust, and the meal carries it back into the kiln. The internal cycle is the invisible yo-yo, its circuit time is of the order of an hour, and a kiln feed suddenly loaded with chloride responds within one shift with coating.
• The external cycle (raw mill & dust circuits): while the raw mill operates on the kiln gas, the moisture and the surface of the raw meal scrub a large fraction of the gaseous sulfur and alkali from the flow, and the mill filter dust, enriched in the condensed salts, returns to the raw mill feed. When the raw mill stops, this scrubbing is lost and its sodium dispersion with the conditioning tower: the well-known observation that preheater blockages cluster in the raw-mill maintenance days has this cause.
• The fuel cycle: the fuels (coal, petcoke, alternative fuels) deliver sulfur, chloride and alkalis of their own; the alternative fuels with their variable chloride content are the modern driver of the bypass designs, because the average that looks acceptable hides the daily spikes.

The “pretreat dust return” is the mechanical companion of the external cycle: the preheater filter dust, fed back to the kiln feed or the raw meal, may carry the very alkalis the gas has just released, and the routing decision (kiln feed vs. silo vs. bypass) is a process decision, not a housekeeping decision. The three-cycle diagram from the package hangs on the wall of many control rooms; it is the circuit diagram of the volatile chemistry.

3. The Chemistry of the Deposits: Type, Place and Fingerprint

Each accumulation that blocks the equipment has a documented chemistry and a documented temperature window:

• The sulfate-rich build-ups in the kiln inlet and the lower cyclones: at 800–950°C the potassium sulfates cement the dust into hard, dense layers. The reducing zone decomposes the sulfate and suddenly raises the circulating sulfur, which is why the reducing flame conditions are the classic trigger of the sulfatic blockages: the reducing kilns ring sooner, the oxidizing kilns last longer, all other things being equal.
• The chloride coating of the top stages: the chlorides condense freely below 800°C, in the cold cyclones and the vertical ducts: the sticky white paste that plugs the cyclones 2 and 3 grows at a raw-meal chloride above 0.01–0.02% and follows the chloride concentration closely. A plugged cyclone stage in a 5,000 t/d line means a kiln stop..
• The kiln-inlet ring (the “sulfur ring”): at the transition zone, the sulfate-rich melt that freezes at the nose: the ring throttles the gas and the meal, the kiln feed backs up, the output falls, and the pressure and the flame whorl let it extend to the new ring harder. The remedies are the bypass, the oxygen discipline, the raw-mix leverage, and the scheduled “ring-burn” operations of the file.
• The cooler snowmen: at the cooler inlet the alkali-and-sulfate-rich fluxes build on the grate plates and grow upward, the classic snowman shape; the cooler air distribution is destroyed and the red clinker spills from the broken grate; the process repeats at every un-vented volatile spike.

The diagnosis of a deposit is the combination of the readings: the differential pressures of the cyclones, the shell scanner thermal maps, the analysis of the removed sample (XRF plus XRD), and the feed and fuel analyses of the days before the event. The file’s identity: the lab sample of a deposit “speaks”: the K2SO4 signature is a sulfur problem recorded, the KCl signature is a chloride problem, the spurrite signature is a burning-zone and load problem, and each signature points to its own correction.

4. The Sulfur-to-Alkali Ratio: The Master Dial

The most practical number of the whole subject is the molar ratio of SO3 to the alkali equivalent (K2O + Na2O as the Na2O equivalent, 0.658 times the K2O). The file presents the behavior in three bands:

• Ratio below 1 (alkali excess): more alkali than the sulfate can bind: the excess alkali circulates as the volatile chloride compounds, the chloride cycle is at its most violent, and the preheater clogging risk is at its highest. The operator’s remedy is to add a sulfur-bearing corrective (a pyritic marl, a sulfate salt or a gypsum addition) so the alkali stands as the non-volatile sulfate, locked out of the circulation;
• Ratio above 1 (sulfur excess): the excess sulfur leaves the sulfate melt and the SO2 emission rises: the sulfate rings appear at the kiln inlet, the resistance of the kiln feed rises, and the environmental limits tighten. The operator adds an alkali-rich clay or plays the fuel ashes to bring the ratio down;
• Ratio near 1: the alkali and the sulfur meet in the potassium-sodium double sulfates that finish immovably in the clinker: the plant runs with the least circulation and the least coating: the “sweet spot” that some plants deliberately build by the raw-material selection: the file summarizes the whole chapter in the line: the system runs on the pair, not on the members.

The kiln gas oxygen is the second dial of the same control: the reducing pockets (kiln-inlet O2 below about 1%) decompose the sulfates and drive the sulfur cycle: its oxygen window 1.5–3.5% at the kiln inlet is an operating target partly for exactly this reason. The file quotes the classic plant experiment: the same feed, the kiln averaged one preheater blockage per three months when runaway at 1% oxygen, and one per two years when the oxygen was stabilized near 2.5%: the O2 is the second master key of the circulation.

5. The Sulfur Cycle in the Preheater and the SO2 Behavior

The sulfur has its own internal map. The volatile sulfate travels with the gas to the mid and top stages, where the fresh raw meal absorbs it: the SO2 at the preheater outlet of a well-run plant is typically 100–1,500 mg/Nm³ (after the kiln contribution), and the meal absorption captures 55–75% of the sulfur that begins its journey. The absorption depends on the free lime in the meal, the cyclone temperatures and the reaction length, and it collapses when the raw mill stops, which is why the SO2 peaks and the sulfur “burps” of the kiln appear in the maintenance windows.

The second sulfur door is the clinker sulfate content itself. The SO3 of the clinker, normally 0.5–1.5%, grows when the sulfur input is high and the alkali cannot hold it: the clinker sulfate, polished beyond 2%, changes the cement set and attacks the concrete, so the process engineer watches the clinker SO3 as the balance of the whole sulfur system. The file’s sulfur chapter ends with the practical calculator: given the raw meal S and the fuel S, the entering SO3 per tonne of clinker, and the tables give the expected clinker SO3, the preheater ring risk and the required residual, all in one sheet: the match between the fitted numbers and the plant’s real clinker chemistry is the calibration of the year.

6. The Bypass: The Tool of the Balance

When the volatile input cannot be closed by the raw mix and the fuels, the engineering leaves the system a door: the kiln gas bypass. The standard installation takes a slice of the kiln-inlet gas, typically 5–20% (and up to 30–50% in the chloride-cursed plants), into a quench chamber where the hot gas is cooled with the quench air below the condensation temperature of the salts, and dedusted: the separated dust exports the alkali and the chlorine from the system in concentrated form. The bypass, to be precise, removes solid salts; the acceptance numbers are about the fractions for the chloride removal of 30–70% of the take-off, and the heat loss that goes with it (at 10% take-off, some 20–40 of the specific heat) is the price of the stable kiln.

The engineering details of the bypass: the take-off point at the kiln inlet with its movable orifice, the quench airflow that fixes the temperature of 350–450°C, the bag filter suited to the hot and the condensing dust, the disposal of the exported salt, and the extra fan load of the bypassed stream. The bypass does not replace the circulation; it throttles it, and the cyclone differential pressures remain the running gauge of the loop. The interactions are real: the bypassed gas lowers the drying duty of the raw mill, the heat balance of the tower shifts, and the plant records the seasonal split (a larger budget in the raw meal with the hot summer feed, a smaller in the winter) from its own history. The file’s sizing worksheets fold all the interactions into one balance sheet, so the plant proves the required take-off percentage before the contractor mobilizes.

The engineering choices of the design: the take-off point (at the kiln inlet, with the variable openings and the slide gates), the quench air ratio (the temperature target 350–450°C), the dedusting (a bag filter with the corrosion-resistant bags), the dust return policy and the management of the exported salt (the fertilizer line, the disposal), and the harmonic interaction: the bypass does not nullify the circulation above, it reduces it; the preheater cyclone DP remains the running gauge of the loop. The bypass is not a resort of the failures: it is the designed respiratory of the system, the file supplies the sizing tables so the plant can prove its required take-off before the order is placed.

7. The Operational Measures: What the Operator Can Do Today

Before any capital is spent, the operator has a documented ladder of measures:

• Oxygen and flame discipline: avoid reducing conditions in the burning zone, hold the kiln inlet O2 in the 1.5–3.5% window, and raise it by 0.3–0.5% when the sulfur load rises: the reducing events the sulfate melt more than any other dial;
• Temperature management of the preheater: watch the differential pressure of every cyclone as the blockage watchdog: a rising DP of one cyclone announces the plug within about an hour; act with the pulse-cleaning, the aspiration or the cooling change before it hardens;
• The raw mix leverage: steer the sulfur-to-alkali ratio toward the 0.8–1.2 window with the available natural materials; the correction goes through the bed and the raw mill, not through the kiln feed;
• Feed and fuel discipline: every significant feed and fuel change in stages: the “volatile shock” of a chloride-rich lot or a massive fuel switch is the classic trigger of the worst blockages; the kiln wants the new inputs in little steps;
• Planned unplugging: the emergency plan of the preheater blockage is a house instruction: the safe access, the CO2 and the anti-dust precautions, the roles of the team: the file provides the complete checklist.

The good operator lives with the circulation: he knows the pressure signature and the ring history of his own kiln, and he treats every new fuel contract and every quarry theater as a potential contributor to the loop. The files that he keeps: the balance spreadsheets, the deposit samples register and the events log, are the three books of the system. The emergency book on the same shelf holds the preheater unplugging procedure, the CO2 purging and the roles of the rescue team, because when the cyclone differential climbs, the documented procedure is what converts the event into a controlled shift instead of a multi-day outage: the discipline of the pre-gold rules is the cheap insurance of everything this article describes.

8. The Monitoring Program: The Balance, the Fingerprints and the Habits

The professional monitoring of the volatile chemistry is a fixed calendar:

• The weekly analyses of the raw materials and the fuels: chloride, sulfur, alkalis, on the composites of the sampling: the chemistry of the quarry flank is a process variable, not a geology report;
• The monthly balance of the kiln system: feed, fuel and bypass in; clinker, dust and exit gas out: the unwarwicked discrepancy is the measure of the accumulation: the balance closes when the system runs clean;
• The deposit museum: every blockage sample, ring sample and snowman sample is analyzed (XRF + XRD) and registered: the fingerprint of the deposit names its own cycle, and the plant that keeps the register sees the tendencies of the seasons;
• The event correlation: the line of the events, the O2/CO and SO2 of the gas analysers, and the coating pictures of the towers: the match between the chemistry and the logs is the basis of the monthly operating review.

The balance method is exactly the book keeping of the introduction: the numbers close monthly, and the unexplained figure is the truth the plant faces: the file provides the templates, the working tables and the benchmark ranges, so the plant that follows the discipline makes the circulation its own numbers and its own decisions, and the kiln becomes predictable again.

9. The Limits and the Benchmarks: Tables for the Designer and the Operator

The file’s reference tables condense the whole discipline into the numbers that the designers and the engineers quote in their reports. The working values, on the basis of the kiln feed or the clinker equivalent as indicated:

Parameter	Unit	Recommended limit	Consequence above the limit
Chloride in the raw meal (no bypass)	% Cl	0.015–0.025	Preheater chloride coating, blockages
Chloride in the clinker	% Cl	0.02–0.06	Rebar corrosion risk in concrete
Alkali equivalent of the raw meal	% Na2O-eq	1.0–1.5	Circulation, coating, cement quality
Alkali in the clinker	% Na2O-eq	0.5–1.2 (product dependent)	Alkali-aggregate reaction in concrete
Sulfur/alkali molar ratio	SO3/(K2O+Na2O)	0.8–1.2	Rings, SO2, chloride churn per the band
Kiln inlet O2 (gas)	% O2	1.5–3.5	Reducing events mushroom the S cycle
Clinker SO3	% SO3	0.5–1.5	Set behavior; process upsets above 3%

These numbers are starting points, not absolutes: a plant with the wet-process history of very different dust cycles, or an alternative-fuel program rich in chloride, shifts the safe window significantly, and the file teaches that each plant must build its own history first. The reason the numbers are quoted at all is that the operators need the alarm values written on the control-room sheets before they become emergencies; the benchmark tables give the alarm setting of the first day, and the plant’s own events refine them within the quarter. The same tables are also the raw-material tender documents, because the supply contract that states the acceptable chloride and alkali windows of the quarry protects the kiln more than any later correction could: the quality clause of the quarry contract is the cheapest circulation investment available.

10. The Case Histories of the File: The Kiln That Was, the Kiln That Is

The file closes its technical chapters with the anonymized case studies that the operators learn under: the plant with the sulfate ring that lost 3% of availability until the raw mix gained the last 0.3% oxygen and the alkali balance shifted 0.9; the chloride plant whose preheater plug was the alternated fuel-day and the kiln recovered only after the bypass of 12% was commissioned and that the “summer phenomenon” of the chlorine spikes disappeared; and the classic ally of the file: the plant that mistook for a refractory problem a successive alkali attack, spent two refractory generations, and only then measured the clinker alkali and found the 2.1% trend, corrected within the quarter.

The lesson of the history: the circulation phenomena rarely arrive as the chemistry theory; they arrive as the symptom (the rising cyclone pressure, the honey smell of the kiln-inlet ring, the red clinker of the cooler), and the discipline of the file is the discipline that reads back, from the symptoms, to the balance, to the feedstock. The plant report of the month of the event is the starting document of every by-case study, and the package file includes the report templates so the engineer reconstructs the history of its plant before the crisis arrives again.

The circulation phenomena are the one chapter of the cement process where the chemistry textbook and the operator’s diary meet on the same page: the volatility tables of the literature and the pressure curves of the night shift tell the same story, and the plant that speaks both languages is the plant whose kiln remains stable for decades. The file of the package puts exactly that bridge in the engineer’s hands, with the tables, the calculators and the case histories that turn the phenomena into a manageable, bookable process.

The Frequently Asked Questions

What is the difference between an internal and an external cycle?

An internal cycle runs inside the kiln-preheater system only: the volatile vaporizes in the burning zone, condenses in the preheater and rides back with the meal. An external cycle goes through the raw mill and the dust: the tower gas passes the raw mill, the salts condense onto the meal and the dust, and the mill returns them. The two are distinguished by measuring the kiln feed quality with the raw mill running and with the raw mill stopped: the mill adds the external loop, and with it the SO2 sinking and the extra recirculation.

Why do chlorides clog the preheater so much faster than the sulfates?

Because the chlorides are far more volatile: nearly all of them volatilize in the kiln and condense below 800°C, exactly in the cold top cyclones, and the cycle factor is 10–40 times the sulfur factor: the effective preheater concentration becomes many times the feed level, and the sticky condensation paste builds in days or weeks where the sulfates take months.

What is the alkali-sulfur ratio and why is it called the master dial?

It is the molar ratio of SO3 to the alkali equivalent (K2O + Na2O) in the materials and the fuels. Far below 1 means the alkali excess and the savage chloride cycle; far above 1 means the sulfur excess and the SO2/ring problems; near 1 the alkali and the sulfur leave with the clinker, and the balance is small. The plant moves the ratio with its raw materials, and the ratio decides which blockage the plant will fight: the master dial of the system.

When does a plant need a kiln gas bypass?

When the volatile load exceeds what the raw-mix balance and the operations can live with: typically the chloride above 0.015–0.025% in the kiln feed (or the strong chloride of the alternative fuels), the alkali equivalent above the limits with the sulfate unable to compensate, and the recurrent blockages in the fall of the year. The bypass takes 5–20% of the inlet gas (more in the chloride plants), exports the salts with the dust: the sizing is in the file.

Does the circulation affect only the refractory?

No. It degrades the refractory certainly, but it also blocks the preheater and the riser (availability), raises the SO2 and the dust emissions, overloads the filters, disturbs the burn and the clinker qualities, and puts the alkali in the cement, the classic trigger of the alkali-aggregate reaction in the customer’s concrete and the era. The volatiles management is the whole integrity of the kiln in motion.

Does the package include a calculation tool for the cycles?

Yes: the Complete Cement Technical Package includes the cycle-balance Excel with the volatility tables and the bypass-sizing worksheets. The plant enters its monthly feed, fuel, clinker and dust analyses and receives the alkali, sulfur and chloride sheet, the cycle factors and the deposit risk forecast: the 931 files of the pack, the balance of the kiln in one sheet.

This file is part of the Complete Cement Technical Package (931 files) available from cementequipment.org. Respective rights holders; library copy for the licensed single user.