Circulation Phenomena: Complete Technical Guide

Circulation phenomena are the invisible chemistry inside the kiln system: the elements that evaporate in the high-temperature zones and condense in the cooler parts, circulating again and again between the preheater and the burning zone: alkalis (sodium and potassium), sulfur, chlorine, and the trace metals: these elements do not leave with the clinker alone: they accumulate in the system, crystallize on the walls, form the rings in the kiln and the build-ups in the preheater, choke the riser ducts and the cyclones, change the flame and the coating, and silently steal the production and the fuel: the circulation phenomena are the explanation behind most of the “unexplained” troubles of the clinker lines.

The Complete Cement Technical Package (931 files including the process chemistry books, the kiln operation manuals, the Excel balance tools and the case databases: $249.99 one-time: instant download via the PayPal payment) includes this complete file on the circulation phenomena in the clinkerization process: this article walks the file: the volatile elements and their behavior, the volatilization and the condensation, the circulation factors and the enrichment of the build-ups, the mechanisms of the sulfates and the alkalis and the chlorides, the consequences for the kiln operation, the materials and the fuels, the laboratory investigation methods, the countermeasures (the bypass, the fuel/raw changes, the chlorine limits) and the troubleshooting tables: the reader finishes with the ability to diagnose the circulation problems of his own plant.

The subject is important because the clinker process is a closed loop: everything that enters the kiln from the raw material and the fuel must leave either with the clinker or with the gases and the dust: the volatile elements try to leave through the gas path, but the gas pair enters the same system over and over: the result is the circulation: the concentration of these elements, balanced at the values dictated by the operating conditions: the modern plants know the maximum tolerable values of chlorine (about 0.015-0.025% in the kiln feed) and the sulfate ratio (the SO3/Alkali ratio of 0.8-1.2) beyond which the cake and the build-up begin: this article explains why these numbers exist and what the plant does when they are exceeded.

1. The Volatile Elements: Who They Are and Where They Come From

The elements of the circulation in the clinker process:

• The alkalis: the potassium (K2O) and the sodium (Na2O) of the clays, the feldspars and the sludge; typically 0.3-1.2% in the raw materials;
• The sulfur: the SO3 of the pyrite and the gypsum in the raw materials, and the fuel sulfur of the coal (0.5-3%) and the petcoke (3-7%): the modern high-sulfur fuels intensified the sulfur circulation;
• The chlorine: the chloride of the chlorides and the organic matter: 0.01-0.1% in the raw materials, the huge attraction of the chloride cycle; and the chlorine of the alternative fuels (the plastics, the household waste) may reach the plant via the AF;
• The heavy metals: the lead, the zinc and the cadmium of the trace minerals and the dust; less known, they also circulate and enrich the kiln dust, the bypass dust and the filter dust;

Element	Evaporation temp. approx.	Typical % in the raw meal	Main vehicle
K and Na (alkali)	700 – 1,100 C	0.3 – 0.9 as K2O+Na2O	the clays, feldspar
S (sulfur)	900 – 1,200 C as sulfate	0.1 – 0.8 so3	pyrite, gypsum, fuel
Cl (chlorine)	600 – 900 C (chloride)	0.01 – 0.05	chlorides, organics
Pb / Zn	700 – 1,200 C	traces (ppm)	trace minerals

The threshold thinking: every kilogram of the volatile element that enters the system circulates until it leaves: if it leaves with the clinker at the 60% and re-evaporates at the 40%, the internal circulation is 2.5 times the feed (the factor 0.4/(1-0.6)=1.0), and the build-up is the surplus: the file quantifies the fractions of the volatilization and the deposition for each element with the measured data of the kilns.

2. The Cycle of the Alkali: The Evaporation and the Condensation

The alkali metals (the potassium and the sodium) enter the burning zone in the raw meal: part reacts into clinker minerals (the C3A-K3A, the C4AF with the Na2O to the dipole), and the rest evaporates in the burning zone at the temperatures of 1,300-1,450 C:

• The volatilization: in the flame zone and the burning zone the temperature above 1,300, the alkali sulfates and the chlorides evaporate: the volatilization rate of the alkali in the well-run kiln is 50-80% of the input (used for the balance; the rest in the clinker);
• The condensation: the gas leaves the burning zone and cools through the kiln transitional, the calciner and the preheater: below the condensation temperature of the alkali sulfate (about 1,100), the vapor condenses on the dust particles and the walls: the fine, sticky particles arrive in the preheater;
• The return: the dust that carries the condensed alkali returns to the kiln with the meals and the recycle dust: the alkali deposits on the kiln nose: the cycle of the loop: evaporation, condensation, deposition, circulation:
• The consequences: the alkali accumulation in the internal loop arrives at the equilibrium of the concentration: the kiln feed the analysis: K2O 0.6, the feed of the preheater with the internal deep the double: the consequences in the clinker the quality (the alkalis in the cement) and the build-up;

The alkali balance equation of the file: the alkali input (raw + fuel + dust) = the output (in the clinker + the bypass + the dust removed): the operability of the kiln is decided by the “alkali load”: the modern plants keep the alkali of the clinker below 0.6-0.9% and the alkali/sulfur ratio in the safe window, with the table of the file giving the classification: the S/O3 temporary, the molar ratio of (SO3/K2O): the operability: below 0.5 “alkali-rich” with the coating, above 1.5 “sulfur-rich” with the buildup and the kiln gas.

3. The Sulfur Cycle: The SO2, the Sulfate and the Reduction Oxidation

sulfur enters the kiln system as the sulfates of the raw meal and the fuels, and its cycle is a chemical drama between the forms:

• The raw materials: the pyritic and the organic sulfur of the limestone and the marl; the fuel sulfur of the coal and the petcoke: the kiln inputs 1-3 kg of SO3 per ton of the clinker in the typical;
• The oxidation: in the burning zone (the oxidizing atmosphere, 1,300-1,450 C) the sulfates are stable in the clinker
• The reduction: in the reducing zones of the kiln (local the flame cover) or the calciner with the CO, the sulfate decomposes the SO2 sulfur gas tha leaves the kiln to the preheater
• The condensation: the SO3 in the gas condenses as the sulfates on the particles of the preheater and mixes with the alkalis: the accumulation on the cyclone walls grows the yellow-white solid build-up

The sulfur cycle is always accompanied by the alkali cycle: the two are chemically married; the sulfates crystallize the K2SO4, the 3K2SO4 reactions: the both “cycle together”, and their relation determines the condensation: the file discusses the ratio of the SO3 to the alkali: the plants operate crowded in the window where the SO3/K2O+Na2O on the feed is 0.7-1.5 (the chloride stack) and the SO3 of the clinker at the same range 0.7-1.2.

4. The Chlorine: The Most Aggressive of the Cycles

Chlorine is the smallest input and the biggest headache: the chloride evaporates virtually completely in the kiln and condenses at the lowest temperatures, forming the sticky potassium chloride:

• The volatility: the chloride salts (the KCl, the NaCl of the calcium chloride) evaporate already at 700-900 C: the volatilization 90-100%, so the cycle of the chloride is nearly closed:
• The enrichment: the chlorides condense at the lowest temperature of the system, the first stages of the preheater (the cyclone I-II at 300-450 C): the concentration there can reach the 20-50 times the input:
• The hazard: the deposits of the chloride are sticky, the build-ups of the cyclones with the shortening of the flow, the holiday block of the cyclones (the plaster), the salt storms and the kiln feed plugs: the plant with the feed chlorine of 0.05% fights the two-week problems:
• The “chloride fraction”: the max practical: the Cl in the kiln feed up to 0.02-0.05% depending on the system: the modern limit of the operation: the bypass rate proportional:

Chlorine in the kiln feed	Expected behavior	Typical countermeasure
under 0.02	manageable, low build	none
0.02 – 0.05	build at the preheater risers, the cyclone	the bypass 1-5%, the shutdowns clean
above 0.05	frequent blockages, the unsafe operation	the bigger bypass, the feed changed

The file describes the chloride measurement methods (the wet chemistry and the XRD/ the XRF of the deposits) and the “chloride fingerprint” of the restricted kilns: the simple preventive method: the weekly chloride analysis of the fly dust and the build-up, so the problem is treated at the concentration of the month, not at the blockage of the week; the plants of the package keep the table of the chloride trend and the triggers.

5. The Lead and the Zinc Cycles: The Trace Metals That Matter

Beyond the big three, the trace metals of the lead and the zinc run their own cycles, and the modern plants with the alternative fuels watch them closely:

• The volatilization: the lead and the zinc evaporate at 1,000-1,400 C in the burning zone: the lead volatilizes 40-80%, the zinc 10-40% under the oxidizing:
• The condensation: they condense with the alkalis and the chloride in the preheater, the analysis of the cyclone dust shows the enrichment of the Pb and the Zn at 10-50 times the feed:
• The consequences: the build-up of the kiln inlet (the “lead rings”), the coating on the calcinator nozzles, and the enrichment of the bypass dust that changes the classification of the dust (the hazardous classification in the EU and the corresponding disposal limits):
• The monitoring: the quarterly analysis of the bypass dust and the filter dust for the Pb, the Zn and the Cd: the limit of the Pb in the dust for the landfill and the reuse in the cement is regulated in the many countries:

The file treats the trace metals as the part of the complete circulation balance: the tables of the vapor pressures, the enrichment factors and the dust classification: the engineer of the plant with the alternative fuels reads this chapter first, because the chlorine and the trace metals of the waste fuels are the difference between the profitable co-processing and the chronic build-ups.

6. The Build-ups and the Rings: The Visible Face of the Circulation

The circulation phenomena become visible in the shapes of the deposits, and each shape has its chemistry and its location:

• The preheater build-ups: the deposits in the cyclone cones, the riser ducts and the calciner: the sulfates, the chlorides and the alkali: the yellow-brown hard crusts; the symptoms: the rising pressure drops, the loss of the circulation, the blockages;
• The kiln rings: the annular deposits inside the kiln: the mid-kiln (the 20-40% of the length) with the sulfate-rich rings (the sulfur), and the kiln inlet rings (the chloride and the sulfate): the rings reduce the kiln diameter, slow the material, raise the kiln back-end temperature and finally block;
• The nose and the cooler build-up: the deposits on the kiln nose ring, the cooler roof and the grate: the “snowmen” of the cooler, the crust on the inlet: the clinker fluid has the material that sticks:
• The alkali “veils” and the curtains: the thin coatings on the chains and the kiln internals that change the heat transfer:

The identification of the deposit chemistry is the key to the remedy: the sample of the build-up analyzed by the XRF (the SO3, K2O, Na2O, Cl, the CaO) tells the operating engineers which element circulates: the file provides the “deposit identification table” that links the location and the analysis to the cause: the diagnosis becomes the prescription: the kiln engineer of the file runs the chemistry of the build-up like the pathologist of the process.

7. The Circulation Factors: The Numbers of the Loop

The quantitative measures of the circulation, defined in the file:

• The volatilization rate: the percentage of the input that evaporates in the kiln (e.g. the potassium 40-70%, the chloride 90-100%):
• The condensation rate: the share of the evaporated element that condenses inside the system: (the condensation of the potassium 80-95%):
• The circulation factor: the ratio of the element passing the calciner/preheater to the input: the circulation factor of the potassium 1.5-3 in the typical kiln, of the chloride 3-15:
• The mass flow in the loop: the tons of the element per hour circulating in the dust and the gas: the example: the feed potassium 0.5 kg/t cl, circulation factor 2.5: the internal flow of the 1.25 kg/t cl through the preheater:

Element	Volatilization	Circulation factor	Deposition locus
K2O	40 – 75%	1.5 – 3	preheater, kiln inlet
Na2O	30 – 60%	1.2 – 2.5	preheater, dust
SO3	40 – 80%	1.5 – 3.5	mid-kiln ring, cyclone
Cl	90 – 100%	3 – 15	cyclone I-II, riser

The circulation factor is the number that scales the problem: the chloride at the input of 0.03% with the factor 10 means the internal loop of the 0.3% of the chloride — the tenfold concentration in the most condensed zone: the file teaches the measurement of the factors from the sampling of the feed, the cyclone dust, the kiln feed and the clinker, with the balance closing example of the file.

8. The Fuel of the Circulation: The Petcoke and the Alternative Fuels

The fuel quality is the modern driver of the circulation problems:

• The petcoke: the sulfur 3-7%, the low ash: the high sulfur forces the sulfate cycle: the kiln must burn the petcoke with the excess air and the reduced CO to keep the sulfates stable, or the SO2 spikes and the sulfur recirculates:
• The alternative fuels (RDF, the plastics, the biomass): the chlorine 0.5-2% of the RDF: the plastics deliver the chloride and the lead to the process: the chlorine content of the fuel is the limiting factor of the co-processing rate:
• The fuel/raw balance: the total input of the chloride and the sulfur is the sum of the raw and the fuel: the modern plants compute the “chlorine budget” and the “sulfur budget” per week and schedule the alternative fuel share accordingly:
• The substitution rate: the thermal substitution rate (TSR) of the AF 30-80% of the modern kilns with the bypass and the chloride control:

The economic significance: the waste fuels replace 20-60 USD/t of the petcoke/coal, but the chlorine and the metals of the fuels create the operating cost of the build-up cleaning and the bypass dust disposal: the file presents the net-benefit calculation: the TSR of the plant with the raw chloride 0.02% and the RDF of 1.5% chlorine is limited to the 40-50% before the bypass becomes the money: the tables of the file are the instrument of that decision.

9. The Bypass System: The Safety Valve of the Circulation

The bypass (the kiln gas extraction) is the classic countermeasure of the volatile circulation: a side stream of the kiln exhaust gas (typically after the kiln inlet) is quenched, dedusted and removed from the system:

• The principle: the gas of the kiln inlet carries the volatile vapors: extracting 1-10% of the gas removes the proportional share of the circulating elements: the bypass rate is the knob of the loop:
• The hardware: the gas extraction duct at the kiln inlet (the “side draft”), the quench air (the rapid cooling prevents the sticking of the duct walls), the cyclone/ESP and the separate dust disposal:
• The rates: the bypass 2-5% of the kiln gas covers the chloride and the alkali control in the typical: the high-chloride plants up to 10-15%: the energy penalty of the bypass (the fuel for the evaporated part and the dust removal) about 10-20 kJ/kg of the clinker per percent of the bypass:
• The dust of the bypass: the enriched dust (the K, S, Cl, Pb) is used in the cement grinding in the small doses (the sulfate correction) or disposed: the file covers the dust management and the quality of the cement with the bypass dust:

The sizing of the bypass is an engineering discipline in the file: the mass balance of the volatile elements with the bypass rate, the expected Cl and SO3 reduction in the kiln feed, the effect on the preheater build-up and the cement quality: the example of the file: the raw with 0.04% Cl: the bypass of 5% brings the effective Cl load of the kiln feed under the 0.025% and the blockages of the preheater disappear: the same arithmetic serves the plant that plans the new bypass or the expansion of the existing.

10. The Countermeasures of the Daily Operation: The Preventive Practice

The file orders the countermeasures from the daily to the strategic:

• The raw materials management: the blending of the high-alkali and the high-chloride layers of the quarry, the stockpile segregation and the control of the kiln feed chemistry: the first line of the defense:
• The stable burning: the reducing zones in the kiln are the amplifier of the sulfur circulation: the stable flame, the excess O2 of the kiln tail 2-4%, the CO under 1%: the file explains the relation with the SO2 and the sulfate stability:
• The cleaning schedules: the planned removal of the build-ups at the stops: the maintenance, the sandblasting and the mechanical cleaning of the preheater cones: the file gives the cleaning standards and the frequencies of the plants with the history:
• The “mild” period: the operation in the reduced load with the stable feed when the build-up signal (the pressure) appears: the relief sequence of the file: the operator waits, reduces the load, and the circulation falls

The preventive practice is the combination of the chemistry control and the discipline: the file quantifies the effect of each measure on the circulation factor, so the plant chooses the cheapest control that works: the frequent result: the raw blending and the stable flame solve the majority of the build-up problems without any hardware, and the bypass remains the reserve for the high loads.

11. The Investigation of the Circulation: The Sampling and the Balance of the Plant

The plant investigates its circulation problems with a structured campaign, and the file provides the complete methodology:

1. The full sample set: the kiln feed, the cyclone I-IV dust, the kiln inlet gas dust, the clinker, the bypass dust and the filter dust: all sampled at the same operating hour and analyzed for the K, Na, S, Cl, Pb, Zn:
2. The material balance: the input (raw + fuel + dust) against the output (clinker + bypass + dust): the balance closes at 95-105%: the file includes the Excel of the balance:
3. The enrichment profile: the volatile content of the cyclones drawn versus the temperature: the location of the condensation peaks reveals the zone of the build-up risk:
4. The diagnosis: the comparison of the measured with the design limits (the Cl of the kiln feed, the SO3/alkali ratio): the identification of the binding element and the loop:

The investigation of the file leads the plant through the diagnosis in the 3-4 days and ends with the action list: the raw correction, the bypass opening, the fuel change: the sample of the completed campaign in the file shows the whole picture: the alkalis at the cyclone III, the chloride at the I-II, the sulfur in the ring of the kiln, and the corresponding set of the measures: the knowledge becomes the document of the plant that the engineers revisit each year.

12. The Circulation and the Cement Quality: The Alkali in the Cement

The circulation is not only the operating problem: it decides the chemistry of the cement that leaves the plant:

• The alkali in the clinker: the potassium and the sodium of the clinker remain in the cement (0.3-0.9% as the equivalent Na2O): the alkali- sulphur reactions in the concrete, the alkali-aggregate reaction (the AAR) with the reactive aggregates:
• The sulfate balance: the SO3 of the cement is regulated (the EN 197 for the OPC) and the clinker SO3 from the sulfur cycle: the optimum SO3 of the cement for the strength is about the 3.0-3.5% total:
• The standard requirement: the alkali limit of the cement 0.6% equivalent Na2O for the low-alkali markets: the plants that serve the alkali-sensitive projects run the bypass or the low-alkali raw:
• The chloride in the cement: the chloride of the cement is limited (0.05-0.1% per the standards) for the reinforced concrete: the chloride of the raw and the fuels must be controlled at the source:

The file links the circulation balances with the quality limits: the same bypass that saves the preheater from the build-ups serves the low-alkali cement production: the file gives the calculation of the alkali in the clinker from the raw and the fuel, and the operating corridors that satisfy both the operability and the standard of the product: the circulation management is the quality management of the kiln, seen through the volatile elements.

13. The Frequently Asked Questions

What is the maximum chlorine in the kiln feed?

The practical limits of the industry: under 0.02% chlorine in the kiln feed for the trouble-free operation without the bypass; 0.02-0.05% with the bypass 3-8%; above 0.05% the operation becomes chronic and the kiln feed must be corrected: the exact number depends on the system (the preheater stages, the calciner, the bypass) and the file explains the calculation of the own limit of each plant with the circulation factor.

Why does the mid-kiln ring form and how is it removed?

The mid-kiln rings are the sulfate-rich deposits that form at 800-1,100 C in the transition zone where the material flow slows and the condensates meet the dusty gas: the removal: the stable operation (the constant burning zone) prevents; the ring is removed by the careful temperature cycling of the kiln, the mechanical removal in the extreme stops and the stabilization of the fuel: the file details the ring removal procedure with the sequence of the day.

How does the petcoke affect the circulation?

The petcoke brings the sulfur of 3-7%: the sulfur enriches the sulfate cycle and raises the SO3 of the clinker: with the local reducing conditions the sulfate decomposes to the SO2 and the cycle accelerates: the kiln burning the petcoke needs the higher O2 (2.5-3.5% at the tail) and the stable flame, and the plant checks the SO3 of the clinker daily: the file carries the petcoke operating handbook of the plant.

The preheater cyclones block every few weeks: where do I look first?

The first look is the chloride: the analysis of the build-up material (if the Cl above 5-10% it is the chloride): the second the alkali and the sulfur: the third the operating condition (the false air, the flame, the CO): the file’s decision tree takes the sequence and the plant solves the majority of the blockages with the raw blending and the fuel selection, not with the more cleaning crews.

Is the bypass always necessary for the high chlorine raw?

Not always: the alternatives: the raw blending to the lower chlorine, the fuel change to the low-chlorine AF, the increased dust recirculation control and the careful condensation management: the bypass is the most expensive and the most effective: the file calculates the cost-benefit of the three paths for the plant’s own chemistry and returns the recommendation: the bypass when the chlorine cannot be reduced at the source.

14. The Conclusion

The circulation phenomena are the invisible process within the visible process: the alkalis, the sulfur, the chlorine and the metals turn the kiln system into the chemical loop that punishes the careless and rewards the attentive: the file of the Complete Cement Technical Package explains the phenomena with the numbers, the balances, the build-up identification and the countermeasures, and the plant that masters the circulation runs the kiln with the fewer stops, the lower fuel and the stable cement quality: the volatile chemistry is the final frontier of the kiln operation, and this file is the complete map of the frontier.

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.