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Eriksson, M., Carlborg, M., Broström, M. (2019)Characterization of ring deposits inside a quicklime producing long rotary kilnEnergy & Fuels, 33(11): 11731-11740https://doi.org/10.1021/acs.energyfuels.9b00865

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Characterization of Ring Deposits Inside a Quicklime ProducingLong Rotary KilnMatias Eriksson,*,†,‡ Markus Carlborg,† and Markus Brostrom†

†Department of Applied Physics and Electronics, Thermochemical Energy Conversion Laboratory, Umeå University, SE-901 87Umeå, Sweden‡Nordkalk AB, Kungsangsvagen 22, SE-731 36 Koping, Sweden

ABSTRACT: Ring deposits are common problems in rotary kiln operations. The ring is constantly subjected to thermal andmechanical wear counteracting the growth of the ring. If the ring hardens or if the growth of the ring is too rapid, the kiln needsto be shut down and the ring removed, reducing the operational time and profitability of the process. In the present study, ringdeposits from a limestone fed long rotary kiln producing quicklime was sampled and characterized in detail by SEM-EDS,dynamic rate TG, and XRD analyses. This work identifies three hardening mechanisms active in the kiln, an increaseddensification of the ring deposits near the refractory surface, the formation of calcite and spurrite through carbonation of thering deposits, and the intrusion of molten fuel ash and product into the refractory, resulting in a strong attachment of thedeposit to the refractory surface. The work also concludes that a significant part of the ring deposit has its origin in the fuel ash,contributing to deposit mass and increasing the ring growth rate.

■ INTRODUCTION

Rotary kilns are used in many industrial applications.1 The kilnhas a long cylindrical shape and is in slow rotation duringoperation. A small inclination of the kiln forces the feedmaterial down through the kiln during the rotation. In thelower end the fuel and combustion air enter the kiln. The kilnoperates in a countercurrent mode, as the condensed feedmoves down toward the burner and the combustion andprocess gases move in the opposite direction. Rotary kilnprocesses are prone to buildups, and accumulation andcirculation phenomena.2,3 The present study targets theaccumulation of particles, known as ring formation, inquicklime production, see Figure 1. The ring deposit reducesthe effective radius of the kiln, hindering material flows. If thering grows thick enough, the kiln has to be shut down and thering removed.Although ring formation in rotary kiln applications has been

widely studied, for the production of, e.g., ordinary Portlandcement clinker,4−8 calcium−aluminate cement clinker,9

magnesite,10 iron pellet,11−13 sponge iron,14 ferro-nickelalloys,15 and lime mud reburning,16−19 only a few studies onring formation in limestone feed rotary kilns for quicklimeproduction can be found.20,21 Although the principle of therotary kiln remains the same, the process conditions and rawmaterial properties differ significantly between these processes.Ring formation has been attributed to unstable operating

conditions, fuel and ash properties, combustion conditions,feed material properties, accumulation phenomenon, andchemical reactions.22−27 Early detection measures, such askiln shell temperature measurements,28 and countermeasures,such as feed material properties, chemical additives, andphysical removal technologies, are continuously investi-gated.9,29−32

The raw material of quicklime is limestone, an abundantrock that is quarried or mined. The limestone, rich in calcium

carbonate, is heated to decomposition temperature to producethe quicklime, rich in CaO, according to reaction 1.

→ +CaCO (s) CaO(s) CO (g)3 2 (1)

Quicklime is used in many industrial applications and isavailable in a wide range of product qualities.33,34 Thequicklime quality is dependent on the raw material properties,kiln type, fuel properties, and process conditions duringproduction.In the present study, the ring formation in a 150 m rotary

kiln fed at 38−40 t/h with different limestone of high andmedium purity was studied; see Table 1. The feed fractionsrange from 6 to 40 mm. The kiln is fired with coal and wastederived fuel oil. The temperature profile inside the kiln ismeasured as gas temperatures in the preheating zone at 139 mand in the calcination zone at 113 m; see Figure 2. Thepreheating zone temperatures range from 560 to 697 °C, withan average temperature of 615 °C. The calcination zonetemperatures range from 800 to 915 °C with an averagetemperature of 851 °C. At the lower end of the kiln the exittemperature of the product is measured. The kiln operates wellabove the calcination temperature of calcite, and when exitingthe kiln to the cooler, the measured product temperatureranges between 1311 and 1500 °C, with an averagetemperature of 1425 °C. The product is thoroughly calcinedwith a residual carbon content < 0.08 wt %, corresponding to<0.7 wt % CaCO3, and no unreacted cores can be detected.The flame operating temperature is estimated to 1700−1800°C on the basis of adiabatic flame temperature calculations forthe fuels and the operational excess air ratio. For the periodinvestigated the coal had on average 9 wt % ash as received.

Received: March 23, 2019Revised: September 23, 2019Published: September 23, 2019

Article

pubs.acs.org/EFCite This: Energy Fuels 2019, 33, 11731−11740

© 2019 American Chemical Society 11731 DOI: 10.1021/acs.energyfuels.9b00865Energy Fuels 2019, 33, 11731−11740

This is an open access article published under an ACS AuthorChoice License, which permitscopying and redistribution of the article or any adaptations for non-commercial purposes.

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The main components of the coal ash were on a dry basisaverage; SiO2 = 50 wt %, Al2O3 = 24 wt %, Fe2O3 = 10 wt %,CaO = 5 wt %, K2O = 2 wt %, MgO = 2 wt %, and S = 2 wt %,the rest being minor components at concentrations <1 wt %.The fuel oil, used only occasionally and at low feed rates,contained only minor amounts of ash. Chemical equilibriumcalculations show that the fuel ashes are completely melted atthe operating flame temperatures. Kiln shell temperatures wereused to localize the position of the rings, and an extensivesampling campaign was performed during a kiln shutdown.

■ METHODS AND MATERIALSSampling and Sample Preparation. The kiln was shut down

and emptied for maintenance. When the kiln was cool enough toenter, samples from the deposits were gathered manually. Extensivekiln shell temperature measurements were used to position the area atwhich the most deposit formation usually occurs. Five samples fromthis area were selected for extensive analysis, 11, 14, 19, 21, and 24 m;see kiln drawing in Figure 2. Ocular inspection showed a verticallylayered deposit structure consisting of fine particles. The layers wereof different texture and color. Since the samples were too big toanalyze as such manual division into subsamples for analysis wasperformed on the basis of the visually identifiable layers. Thesubsamples will in the following be referred to as zones. All zones

were consistently numbered starting from the refractory surface, zone1, toward the center of the kiln. Flat surfaces were used to identify thecontact layer with the refractory; see Figure 3. The kiln has differentrefractories. The refractories at the sampling points are of threedifferent qualities and consist of 91.0−93.2 wt % MgO, 4.5−5.5 wt %Al2O3, and <1 wt % CaO, Fe2O3, and SiO2, respectively.

Deposit samples were sectioned by dry cutting with a rotatingdiamond disc. To not overheat equipment and samples during drycutting, it was made in short intervals and with removal of debris withcompressed air. For scanning electron microscopy−energy dispersiveX-ray (SEM-EDS) analysis, samples were cast into epoxy, plane-ground, and polished with SiC paper in dry conditions. Compressedair was used for removal of debris. Pulverization prior to XRD andthermogravimetric analysis was made in a ceramic mortar; in the caseof hard components a steel mortar and pestle were used to crush thesample.

Scanning Electron Microscopy and Energy Dispersive X-rayAnalysis. Scanning electron microscopy and energy dispersive X-rayspectroscopy were used to investigate morphology, and elementalcomposition and distribution. A Zeiss EVO LS15 with LaB6 electronsource and operated in extended pressure mode for charge removalwas used. Mainly backscattered electrons (BSE) were used forimaging as to benefit from atomic number contrast. Elementalcompositions are displayed as atom percent on a carbon- and oxygen-free basis.

Figure 1. Top: Schematic illustration of rotary kiln producing quicklime. The kiln has a small inclination, Θ, which forces the limestone feed downtoward the flame as the kiln rotates along its longitudinal axis, x. The calcined limestone, the quicklime product, exits the kiln to a cooler designedfor heat recuperation. Bottom left: Ring formation in the rotary kiln in question, as seen from the lower end. Photographed during the short kilnstop for ocular ring situation inspection, with the burner in the foreground and the materials still glowing hot. Bottom right: Schematic illustrationof the ring deposit. The ring reduces the effective radius, r, of the kiln. Initially the reduced radius, r′, forms a dam hindering the material flows inthe kiln. Eventually the kiln has to be shut down and the ring removed.

Table 1. Classification of High Calcium Limestone (wt %)35−37

classification CaCO3 CaO MgO SiO2 Fe2O3

very high purity >98.5 >55.2 <0.8 <0.2 <0.05high purity 97.0−98.5 54.3−55.2 0.8−1.0 0.2−0.6 0.05−0.1medium purity 93.5−97.0 52.4−54.3 1.0−3.0 0.6−1.0 0.1−1.0low purity 85.0−93.5 47.6−52.4 >3.0 <2.0 >1.0impure <85.0 <47.6 >3.0 >2.0 >1.0

Figure 2. Kiln drawing of the investigated lime kiln with horizontal kiln positions in meters. Support tires at 8.5, 32.1, 58.3, 85.9, 113.5, and 139.9m. Samples were collected along the kiln, and samples from 11, 14, 19, 21, and 24 m were selected for extensive analysis.

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Powder X-ray Diffraction. Powder X-ray diffraction (XRD) wasperformed on pulverized deposit samples to investigate crystallinecompounds. It was performed on a Bruker AXS d8 advance operatedin 2θ mode with Cu Kα radiation and spinning sample. Semi-qualitative quantification of XRD results was obtained with Rietveldrefinement of identified structures with crystal data from the InorganicCrystal Structure Database.38

Dynamic Rate Thermogravimetric Analysis. Dynamic ratethermogravimetric analysis (DRTG) was performed with TAInstruments TGA Q5000 IR. The experimental settings were asfollows: 10−20 mg of sample, CO2 atmosphere, and alumina crucible.

The instrument settings were as follows: maximum heating rate of 50°C/min to 1000 °C, with a dynamic resolution setting of 5.0 andinstrument sensitivity setting of 3.5.

■ RESULTS

Scanning Electron Microscopy and Energy DispersiveX-ray Analysis. Upon visual inspection, the depositsdisplayed a layered character, which was revealed as zones ofdifferent porosity when viewed with SEM. As can be seen inFigure 3, the bottom layer (zone 1) is denser while the upper

Figure 3. Photograph of 19 m sample and SEM image. Left: deposit sample (≈10 cm × 10 cm), with zones and the refractory−kiln centerorientation marked. Right: SEM image (≈1.5 cm × 3 cm), position marked as black rectangle in left image. The sample shows a layered structurethat is denser toward the refractory surface. White rectangles indicate two of the positions further investigated: lower rectangle, in Figure 4, andupper rectangle, in Figure 5.

Figure 4. BSE image and elemental maps for Mg, Al, Si, and Ca of zone 1 sampled at 19 m (corresponds to bottom white rectangle in Figure 3).The sample is dominated by Ca with some Si, low concentrations of Mg, and Al and has traces of Na, K, Fe, P, S, and Cl. EDS analysis indicateslime, alite, and an Al rich phase.

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layer (zone 2) has a more porous structure where theindividual grains are connected via necks. An example of adense zone can be seen in Figure 4, with the elementalcomposition presented in Table 2. The sample is dominated by

Ca, Si, Mg, and Al. The main areas comprise Ca (94 at. %) orCa and Si (73 and 22 at. %). Small areas of high concentrationsof Al and Mg are found. An example of a porous zone is shownin Figure 5 with the elemental composition presented in Table3. The sample is dominated by Ca, Si, Mg, and Al. The mainareas comprise Ca (94 at. %) or Ca and Si (75 and 19 at. %).Again, small areas of high concentrations of Al and Mg arefound.The MgO refractory was infiltrated with melt. The interface

between deposit and refractory is displayed in Figure 6. Melthas entered the refractory, and the melt is enriched in Mg, ascan be seen from elemental compositions of points 3 and 4 inTable 4. K and Cl were found in high, equal concentrations onthe deposit. The Mg content in the deposit close to therefractory was increased, and it was more evenly distributedcompared to the Mg found far away from the refractory. Thisphenomenon can be related to release of MgO from therefractory, where solid MgO is dissolved into an oxide melt or

into a solid solution with, e.g., 3CaO·SiO2 or CaO. However,the phenomenon can also originate in variations in the kilninput feeds.

Powder X-ray Ddiffraction. The following compoundswere identified by XRD analysis: calcium oxide (CaO), calcite(CaCO3), alite (3CaO·SiO2), belite (2CaO·SiO2), tricalciumaluminate (3CaO·Al2O3), portlandite (Ca(OH)2), periclase(MgO), spurrite ((2CaO·SiO2)2(CaCO3)), and sylvite (KCl).Considering the operating temperatures and material flows ofthe kiln, it is feasible to assume that Ca(OH)2 is a reactionproduct of CaO hydratization, during kiln cooldown, sampling,sample storage at the kiln site, sample transport to the analysislaboratory, sample preparation, and analysis steps. Therefore,all analysis results are corrected for CaO hydratization byassuming all Ca(OH)2 as CaO at kiln operating conditions.Table 5 shows XRD data for the different zones of the sample,and Figure 7 shows the average composition of the ringbuildup at different kiln positions.One sample from 24 m contained about 20 wt % spurrite

and the diffractogram is shown to the right in Figure 8 togetherwith peak positions and their relative intensity for spurrite. Thepeaks below 27° for spurrite have few overlaps with otherphases and are clearly visible in the diffractogram. The Rietveld

Table 2. Elemental Composition (at. %) of Points Indicatedin Figure 4

for given position

element full map 1 2 3 4

Mg 2.7 2.4 1.1 62.9 1.9Al 1.7 0.6 1.6 0.9 20.4Si 12.2 2.2 22.2 3.7 10.2Ca 81.8 94.3 73.3 32.0 65.5other 1.5 0.5 1.8 0.5 2.1

total 99.9 100.0 100.0 100.0 100.1

Figure 5. BSE image and elemental maps for Mg, Al, Si, and Ca from zone 2 in the sample taken at 19 m (corresponds to upper white rectangle inFigure 3). The images show ≈1 mm particles connected with bridges. EDS analysis indicates the minerals lime (point 1), alite (point 2), and an Alrich phase (point 3); see Table 3 for quantification. The magnesium results suggest that the sample is not contaminated with refractory material.

Table 3. EDS Quantification (at. %) for Spots 1−3 andWhole Image in Figure 5

for given position

element full map 1 2 3

Mg 2.8 1.7 1.5 0.9Al 3.7 1.1 2.2 23.2Si 8.5 2.1 19.8 6.7Ca 82.9 94.4 75.2 67.1other 2.0 0.8 1.3 2.2

total 99.9 100.1 100.0 100.1

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refinement of this phase was made with the structure resolvedby Grice.39

Dynamic Rate Thermogravimetric Analysis. Samplesfrom 11 and 19 m were investigated by dynamic rate TG in aCO2 atmosphere. The results show carbonation from 400 °Cup to the decomposition temperature at 900 °C. Even thoughother mechanisms cannot be excluded on the basis of availabledata, the dominant mechanism is most likely the carbonationof CaO to CaCO3. All zones exhibit reactivity toward CO2; seeFigure 9 and Figure 10. Zone 1 of the 11 m sample shows thehighest carbonation capacity with an increase in the ass of 12wt %. Samples from 11 m zone 1 and all the zones of thesample from 19 m show two decompositions, the first at 900°C, related to calcite and the second, at 950 °C, possibly tospurrite.

Figure 6. BSE image and elemental maps for Mg, Al, Si, and Ca of the interface between deposit and refractory. Melt has entered the refractory,and the melt is enriched in Mg, as can be seen from elemental compositions of points 3 and 4 in Table 4. K and Cl was found in high, equalconcentrations on the deposit.

Table 4. Elemental Composition (at. %) of Measured Pointsin Figure 6

at given position

element 1 2 3 4 5

Na 2.3 0.1 0.3 0.5 0.6Mg 12.5 86.9 11.3 11.2 39.7Al 0.6 0.7 0.4 1.3 3.8Si 2.7 2.7 5.3 19.5 17.1Cl 33.8 0.7 0.9 1.1 0.3K 29.3 0.7 0.6 0.9 0.4Ca 18.6 8.0 80.3 64.3 37.5other 0.2 0.3 0.8 1.2 0.7

total 100.0 100.1 99.9 100.0 100.1

Table 5. XRD Analysis of Ring Samples (wt %), Corrected for Hydratization of CaOa

pos. CaO CaCO3 2CaO·SiO2 3CaO·SiO2 (2CaO·SiO2)2(CaCO3) 3CaO·Al2O3 MgO KCl total

11z1 81.72 0.00 15.89 2.01 0.00 0.00 0.38 0.00 100.0011z2 86.75 0.00 0.00 13.25 0.00 0.00 0.00 0.00 100.0011z3 56.30 1.46 8.56 29.18 0.00 4.50 0.00 0.00 100.0014z1 63.35 10.38 6.43 7.53 9.00 3.32 0.00 0.00 100.0014z2 75.06 1.39 2.75 12.07 5.27 3.47 0.00 0.00 100.0014z3 47.35 11.27 17.00 9.01 12.76 2.61 0.00 0.00 100.0014z4 44.87 12.09 7.61 22.18 10.59 1.84 0.82 0.00 100.0019z1 43.66 15.11 13.76 13.08 8.92 4.77 0.68 0.00 100.0019z2 65.03 4.69 11.82 9.76 4.00 3.85 0.85 0.00 100.0019z3 62.77 10.44 8.96 7.24 5.73 3.39 1.47 0.00 100.0019z4 56.28 13.84 15.54 0.00 9.10 3.49 1.75 0.00 100.0021z1 45.89 15.74 16.12 0.00 13.21 7.59 0.58 0.87 100.0021z2 29.67 13.20 15.29 14.87 15.58 9.64 0.76 0.99 100.0024z1 34.43 3.17 30.03 13.71 10.32 7.37 0.97 0.00 100.0024z2 48.04 5.87 20.23 8.38 8.25 8.59 0.63 0.00 100.0024z3 41.40 2.15 22.94 12.86 10.87 8.20 1.59 0.00 100.00

apos. = Kiln position (m), and z = zone of the sample, where z1 is closest to the refractory surface.

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The sample from 19 m zone 2 shows the most complex re-carbonation behavior and was investigated in multiplecarbonation−calcination cycles. The dynamic rate functionwas not applicable to the cooling part of the cycles, and theexperiment was run at dynamic resolution setting = 0, i.e., atconstant heating and cooling rates. The result shows that thecarbonation capacity of the sample decreases over the cycles;see Figure 11. It also shows that only the first cycle has a cleartwo-step decomposition behavior between 900 and 1000 °C.The major decomposition at 900 °C corresponds to thedecomposition of calcite, while the minor decomposition at950 °C is of yet unknown origin, possibly related to thedecomposition of spurrite.

Figure 7. Average composition (mol. %) of ring samples at five kilnpositions, all zones. Number of observations, n = 2−4. Corrected forCaO hydratization.

Figure 8. Left: Diffractograms from ring samples at different positions in the kiln and different zones within the rings together with referencepatterns for detected phases. Right: Sample from the ring at 24 m with high concentration of spurrite and the reference pattern. The peaks below27° for spurrite have few overlaps with other phases and are clearly visible in the diffractogram.

Figure 9. Results from dynamic rate TG in CO2 of samples from 11m, 100−1000 °C.

Figure 10. Results from dynamic rate TG in CO2 of samples from 19m, 100−1000 °C.

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■ DISCUSSIONThe XRD analysis shows a significant variation in the freecalcium oxide (CaOFree) concentration over the investigatedkiln positions. At 21 m, the CaOFree reached only 64 mol %,corresponding to a low 38 wt %. The main contaminationswere silicon and aluminum. Among the silicon compounds, thealite phases (3CaO·SiO2) dominated at the hotter positionsnear the flame, while, at lower temperatures, further up into thekiln, the belite phases (2CaO·SiO2) dominated. In addition, aspurrite phase ((2CaO·SiO2)2(CaCO3)) was identified; seeFigure 12. Since the kiln was operated with high and medium

purity limestone and magnesia refractories, and since novolatilization−condensation mechanism for silica was active atthe operating temperatures of the kiln, the silicon compoundsmust originate from the high SiO2 fuel ash. The results showedthat the fuel ash was incorporated into the ring deposit overthe whole range of 11−24 m. It can be assumed that the ashwas not incorporated only into the deposit but also into theproduct, reducing product quality.Since the limestone rock utilized for kiln feed does not

contain any spurrite, the spurrite found in the deposit musthave formed by carbonation reaction within the kiln. It isknown from other processes such as cement clinker productionthat spurrite is formed in kiln conditions with atmosphereshigh in CO2 and that when formed, spurrite reinforces depositsby producing a mass of interlocking crystals.40 The chemical

analysis performed cannot unambiguously rule out thepossibility that some calcite originating in the limestone rockmight be found in the deposit samples. However, since mostsamples containing CaCO3 also contain 3CaO·SiO2 and sincethe deposits show reactivity toward gaseous CO2, see Figure13, it is unlikely that is the case. The small particles within the

deposit with an estimated retention time in days, not hours, arelikely to have undergone several calcination−carbonationcycles prior to sampling. Carbonation of the deposit increasesthe deposit mass and wear resistance. TG analysis showsdecreased carbonation capacity of the deposit material overmultiple calcination−carbonation cycles, in line with earlierresults related to calcination−carbonation of CaCO3; see, e.g.,ref 41. The formation of carbonated products such as spurriteand calcite hardens the deposit and thereby significantlyincreases the wear resistance of the ring. Kiln operationsshould be stabilized to avoid fluctuations close to carbonationtemperatures.Spurrite is reported to be stable to between 900 and 950 °C,

depending on other components present.8,22,42 Figure 14shows the calculated stability diagram for the system CaO−SiO2−CO2 in the composition range CaO·SiO2−CaO and 0.1bar of CO2 pressure. Thermodynamic data for (2CaO·SiO2)2(CaCO3), spurrite, and (2CaO·SiO2)(CaCO3)2, till-eyite, from Backman43 and for other components from theFactSage 7.2 database.44 For the kiln deposits the results showthat the CaO to SiO2 mole ratio declines further into the kiln,from 0.93 at 11 m to 0.85 at 24 m. In this range tilleyite isformed at 420 °C and spurrite at 450 °C. Spurrite and calcitecoexist up to around 750 °C, where calcite starts todecompose. Above spurrite decomposition at approximately900−950 °C belite (2CaO·SiO2) is stable until alite (3CaO·SiO2) forms at 1350 °C. The results show that the depositcontains both spurrite and alite, indicating temperaturevariation from below 900 °C up to above 1300 °C.The fuel ash will also interact with the refractory. The SEM-

EDS results show the penetration of a Ca−Si−Al oxide meltinto the refractory at 21 m. The chemical composition of thelimestone feed and the refractory conclude that the silicaoriginates mainly from the fuel ash. Calcium is readily availablein the process and most likely originates from the limestonefeed. The refractory at 21 m contains about 5 wt % alumina,and on the basis of these results the origin of alumina in the

Figure 11. Constant rate TG results of sample from 19 m zone 2 in aCO2 atmosphere. The deviation from linear cooling from 1000 to 950°C was investigated with an empty sample pan and concluded to bean instrumental artifact.

Figure 12. Silica containing compounds (wt %) at different kilnpositions. As identified by XRD. Corrected for CaO hydratization.Average of all zones; number of observations, n = 2−4.

Figure 13. Carbon containing compounds (wt %) at different kilnpositions. As identified by XRD. Corrected for CaO hydratization.Average of all zones; number of observations, n = 2−4.

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melt cannot be clearly determined. The result of the meltpenetration, however, is a strong attachment of the deposit tothe refractory. SEM images also show a densification of thesample toward the kiln refractory. The kiln atmosphere, beinghigh in CO2, together with the high operating temperature ofthe kiln, promote this densification of the CaO matrix.45,46

Since it can be expected that the ring deposit builds from therefractory surface toward the center of the kiln, the materialnearest the refractory has a longer exposure to the conditionspromoting densification.Although no pure phases including iron or sulfur could be

detected with XRD, a systematic association of Fe to otherelements could be observed in the EDS maps. The localizationof Fe seems to be strongly correlated to Al with a constantpresence of Ca and in some cases also Si. There are manypossible phases in the CaO−Al2O3−Fe2O3(−SiO2) system.The phase 3CaO·Al2O3 can hold up to 4.5 wt % Fe3O4, andthis would only yield a slight change in the pattern for thisphase in the diffractogram. It can also appear in solid solutionwith ferrite, giving a compound with compositionCa2(AlxFe1−x)2O5, where x can vary between 0.48 and 0.7.40

In the samples where most S was found, it was localizedtogether with K and Ca, probably as sulfates.

■ CONCLUSIONSBuildup of deposits can be expected in all rotary kilnoperations, and the factors influencing the hardening of thedeposit preventing the wear and spontaneous release of thering deposit is of special interest. Detailed analyses of depositsfrom an industrial scale quicklime burning rotary kiln wastargeted in the present study.The deposits are formed at high temperatures where the

molten fuel ash interacts with the refractory and the quicklimeproduct. The present study identifies three hardeningmechanisms active in the kiln. There is an increaseddensification of the ring deposits near the refractory surface.

Fluctuating temperatures give rise to the formation of calciteand spurrite through carbonation of the ring deposits. There isan intrusion of molten fuel ash and quicklime product into therefractory resulting in a strong attachment of the deposit to therefractory surface. However, on the basis of the materialcharacterization in this work, no quantitative assessments canbe made, e.g., the effect of these mechanisms, individually ortogether, on kiln operations and availability, nor which is thedominant mechanism with regard to kiln shutdowns.Furthermore, the results showed that fuel ash is reactive in

kiln operating conditions. Silica in the fuel ash was extensivelyincorporated into the deposits found at 11−24 m, with nounreacted SiO2 found, contributing to the ring deposit massand increasing the growth rate of the ring. The fuel ash canalso be expected to influence quicklime product qualitynegatively.Kiln operation was not stable during the time for ring

buildup, and fluctuating temperatures allow for carbonation ofbelite to spurrite, and calcium oxide to calcite, hardening thering. The ring deposits consisted of only fine particles, and noquicklime or limestone lumps were found, confirming that thedeposits were built-up by accumulation of particles. Thedensification of CaO−CaCO3 is mainly dependent on CO2 inthe atmosphere, the temperature, and residence time ofaccumulated particles in the deposit and cannot be easilycontrolled.Future work should focus on influences from fuel ash and

operational stability. Maximum kiln loads of silica and aluminashould be determined, and incorporated into limestone feedand fuel specifications. Kiln operators should pay specialattention to avoiding fluctuating temperatures within the kiln.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: matias.eriksson@umu.se

Figure 14. High calcium end of CaO-SiO2 system, in mole fraction, at 0.1 bar CO2 atmosphere. (Red) Stability of tilleyite and spurrite. (Blue,dashed) Area of CaO/(CaO + SiO2), in mole fraction, of overall ring deposit compositions (compounds CaCO3, CaO, 2CaO·SiO2, 3CaO·SiO2,and (2CaO·SiO)2(CaCO3), corrected for hydratization), at kiln positions: 0.93, 11 m; 0.91, 14 m; 0.91, 19 m; 0.87, 21 m; 0.85, 24 m.

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ORCIDMarkus Brostrom: 0000-0003-1095-9154NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

Rainer Backman and Dan Bostrom at Umeå University, DuskoBunjevac at Nordkalk AB, and Jan Bida at the Swedish MineralProcessing Research Association (MinFo) are acknowledgedfor valuable discussions and project support. We acknowledgethe facilities and technical support of Chen Coo Lee of theUmeå Core Facility for Electron Microscopy (UCEM) at theChemical Biological Centre (KBC), Umeå University. Fundingfrom the Swedish Governmental Agency for InnovationSystems, Vinnova No. 2015-04541, the National (Swedish)Strategic Research Program Bio4Energy, and the SwedishEnergy Agency is gratefully acknowledged. The funders had norole in the study design, data collection and analysis, decisionto publish, or preparation of the manuscript. The Centre forSustainable Cement and Quicklime Production at UmeåUniversity is acknowledged for supporting this work.

■ REFERENCES(1) Boateng, A. A. Rotary Kilns: Transport Phenomena and TransportProcesses. Elsevier Science, 2011.(2) Backman, R.; Hupa, M.; Tran, H. Modeling the SodiumEnrichment in Lime Kilns. Pulp Pap. Can. 1993, 94 (11), 78−83.(3) Cortada Mut, M. d. M.; Norskov, L. K.; Frandsen, F. J.;Glarborg, P.; Dam-Johansen, K. Review: Circulation of InorganicElements in Combustion of Alternative Fuels in Cement Plants.Energy Fuels 2015, 29 (7), 4076−4099.(4) Holderbank. Rings, Balls and Build-Ups. Materials Technology II,Holderbank Cement Seminar 2000; Holderbank Management &Consulting: 2000; Chapter 4, https://archive.org/details/HolderbankCementEngineeringBook (accessed Feb. 25, 2018).(5) Chromy, S.; Weber, M. The Process of Ring Formation inRotary Cement Kilns. Zem.-Kalk-Gips 1981, 34 (9), 453−457.(6) Nievoll, J.; Jorg, S.; Dosinger, K.; Corpus, J. Sulphur, Spurrite,and Rings − Always a Headache for the Cement Kiln Operator. RHIBull. 2007, 2, 25−38.(7) Palmer, G. Ring formation in cement kilns. World Cem. 1990,538−543.(8) Sylla, H.-M. Investigations on the formation of rings in rotarycement kilns. Zkg Int. 1974, 10, 499−508.(9) Pisaroni, M.; Sadi, R.; Lahaye, D. Counteracting ring formationin rotary kilns. J. Math. Ind. 2012, 2 (1), 3.(10) Subramanyam, A. V.; Gill, B. S. Study of Ring Formations in aMagnesite Rotary Kiln. Trans. Indian Ceram. Soc. 1983, 42 (5), 134.(11) Bandyopadhyay, R.; Gupta, S.; Lindblom, B.; Jonsson, S.;French, D.; Sahajwalla, V. Assessment of ash deposition tendency in arotary kiln using Thermo-mechanical analysis and ExperimentalCombustion Furnace. Fuel 2014, 135, 301−307.(12) Zhong, Q.; Yang, Y. B.; Jiang, T.; Li, Q.; Xu, B. Effect of coalash on ring behavior of iron-ore pellet powder in kiln. Powder Technol.2018, 323, 195−202.(13) Zhu, D.; Zhou, X.; Luo, Y.; Pan, J.; Zhen, C.; Huang, G.Monitoring the Ring Formation in Rotary Kiln for Pellet Firing. InDrying, Roasting, and Calcining of Minerals: Battle, T. P., Downey, J. P.,May, L. D., Davis, B., Neelameggham, N. R., Sanchez-Segado, S.,Pistorius, P. C., Eds.; Springer International: Cham, Switzerland,2016; pp 209−216.(14) Chatterjee, A.; Mukhopadhyay, P. K. Flow of Materials inRotary Kilns Used for Sponge Iron Manufacture 0.3. Effect of RingFormation within the Kiln. Metall. Trans. B 1983, 14 (3), 393−399.

(15) Tsuji, H.; Tachino, N. Ring Formation in the Smelting ofSaprolite Ni-ore in a Rotary Kiln for Production of Ferro-nickel Alloy:Mechanism. ISIJ Int. 2012, 52 (10), 1724−1729.(16) Dhak, J. Ringbildning i Mesaugnar II (in Swedish); VarmesforskService AB: Stockholm, Sweden, 2005.(17) Ellis, M. Managing lime kiln ring formation at Australian paper,Maryvale mill. Fibre Value Chain Conference and Expo: Pulp and PaperBioenergy Bioproducts, Distinction Hotel Rotorua, New Zealand;Appita Inc.: Macleod, Australia, 2014; pp 49−54.(18) Notidis, E.; Tran, H. Survey of Lime Kiln Operation andRinging Problems. Tappi J. 1993, 76 (5), 125−131.(19) Tran, H.; Griffiths, J.; Budge, M. Experience of Lime KilnRinging Problems at Eb Eddy Forest Products. Pulp Pap. Can. 1991,92 (1), 78−82.(20) Bhattacharya, A. K.; Das, P.; Dasgupta, A. K. Physico-chemicalstudies on ring formation in rotary kiln for calcination of limestone.SEAISI Q. J. 1988, 17 (3), 66−71.(21) Potgieter, J. H.; Wirth, W. An investigation into ash-ringformation in a rotary lime kiln. Zkg Int. 1996, 49 (3), 166−171.(22) Goswami, G.; Padhy, B. P.; Panda, J. D. Thermal-Analysis ofSpurrite from a Rotary Cement Kiln. J. Therm. Anal. 1989, 35 (4),1129−1136.(23) Skrifvars, B. J.; Frederick, W. J.; Hupa, M. Chemical-ReactionSintering as a Cause for Lime Kiln Rings. 1992 International ChemicalRecovery Conference: Proceedings, Books 1 and 2, Vol. 75; TAPPI:Peachtree Corners, GA, USA, 1992; pp 161−167.(24) Tran, H.; Mao, X.; Barham, D. Mechanisms of Ring Formationin Lime Kilns. J. Pulp Pap. Sci. 1993, 19 (4), J167−J175.(25) Tran, H. N.; Barham, D. An Overview of Ring Formation inLime Kilns. Tappi J. 1991, 74 (1), 131−136.(26) D’Souza, T. Effect of process variability on ring formation in limekilns. Doctoral thesis, University of Toronto (Canada), 2005.(27) De Brito, C. A. P. B. Ring and build-ups formation in cementkilns: Magnesita cooperation for their solution; Technical article,Magnesita S.A Export Department, 1996.(28) Hirtz, B.; Marshman, D.; Sheehan, C. Kiln Infra-Red Camerasfor Abnormal Situation Detection and Residual Carbonate Control. J-FOR: J. Sci. Technol. For. Prod. Process. 2017, 6 (3), 28−36.(29) Bjork, M. Effects of process variation on ring formation in limekilns. Tappi J. 2005, 4 (6), 20−21.(30) Gorog, J. P.; Leary, W. R. The use of non-detonating carbondioxide systems as a means to control lime kiln rings. PEERSConference 2015: Sustainable Solutions for Our Future; TAPPI:Peachtree, GA, USA, 2015; Vol. 2, pp 1024−1035.(31) Gorog, J. P.; Leary, W. R. Ring removal in rotary kilns used bythe pulp and paper industry. Tappi J. 2016, 15 (3), 205−213.(32) Peltonen, K., Kiln Ring Formation − a review of what (wethink) we know. The Western Canada Black Liquor Recovery BoilerAdvisory Committee, spring meeting, Richmond, BC, Canada; AndritzPulp & Paper, 2015.(33) Dowling, A.; O’Dwyer, J.; Adley, C. C. Lime in the limelight. J.Cleaner Prod. 2015, 92, 13−22.(34) Eriksson, M. Sustainability measures in quicklime and cementclinker production. Doctoral thesis; Umeå Universitet, Umeå,Sweden, 2015.(35) Cox, F. C.; Bridge, D. M.; Hull, J. H. Procedure for the assessmentof limestone resources: Mineral Assessment Report 30; Institute ofGeological Sciences, Natural Environment Research Council:London, 1977.(36) Harrison, D. J.; Adlam, K. A. M. Limestones of the peak, a guideto the limestone and dolomite resources of the Peak District of Derbyshireand Staffordshire, Mineral Assessment Report 144; British GeologicalSurvey, Natural Environment Research Council: London, 1985.(37) Mitchell, C. High purity limestone quest. Ind. Minerals 2011,531, 48−51.(38) Inorganic Crystal Structure Database (ICSD); FIZ Karlsruhe,Karlsruhe, Germany, 1978.

Energy & Fuels Article

DOI: 10.1021/acs.energyfuels.9b00865Energy Fuels 2019, 33, 11731−11740

11739

(39) Grice, J. D. The structure of spurrite, tilleyite and scawtite, andrelationships to other silicate-carbonate minerals. Can. Mineral. 2005,43, 1489−1500.(40) Taylor, H. F. W. Cement chemistry, 2nd ed.; Thomas Telford,London, 1997; p 70.(41) Wang, C. B.; Zhou, X.; Jia, L. F.; Tan, Y. W. Sintering ofLimestone in Calcination/Carbonation Cycles. Ind. Eng. Chem. Res.2014, 53 (42), 16235−16244.(42) Glasser, F. P. The formation and thermal stability of spurrite,Ca5(SiO4)2CO3. Cem. Concr. Res. 1973, 3 (1), 23−28.(43) Backman, R., Personal communication, 2018.(44) Bale, C. W.; Belisle, E.; Chartrand, P.; Decterov, S. A.; Eriksson,G.; Gheribi, A. E.; Hack, K.; Jung, I. H.; Kang, Y. B.; Melancon, J.;Pelton, A. D.; Petersen, S.; Robelin, C.; Sangster, J.; Spencer, P.; VanEnde, M. A. FactSage thermochemical software and databases, 2010−2016 (vol 54, pg 35, 2016). CALPHAD: Comput. Coupling PhaseDiagrams Thermochem. 2016, 55, 1−19.(45) Maya, J. C.; Chejne, F.; Gomez, C. A.; Bhatia, S. K. Effect of theCaO sintering on the calcination rate of CaCO3 under atmospherescontaining CO2. AIChE J. 2018, 64 (10), 3638−3648.(46) Moropoulou, A.; Bakolas, A.; Aggelakopoulou, E. The effects oflimestone characteristics and calcination temperature to the reactivityof the quicklime. Cem. Concr. Res. 2001, 31 (4), 633−639.

Energy & Fuels Article

DOI: 10.1021/acs.energyfuels.9b00865Energy Fuels 2019, 33, 11731−11740

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