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Chemical Engineering and Processing 48 (2009) 950–954

Contents lists available at ScienceDirect

Chemical Engineering and Processing:Process Intensification

journa l homepage: www.e lsev ier .com/ locate /cep

esidence time investigation of a multiple hearth kiln using mineral tracers

. Thomasa,b,∗, D. Grosea, G. Obajea, R. Taylora, N. Rowsonb, S. Blackburnb

Imerys Minerals Limited, Par Moor Centre, Par Moor Road, Par, Cornwall PL24 2SQ, UKDepartment of Chemical Engineering, School of Engineering, The University of Birmingham, Birmingham B15 2TT, UK

r t i c l e i n f o

rticle history:eceived 7 January 2008eceived in revised form 19 December 2008ccepted 6 January 2009vailable online 15 January 2009

eywords:alcination

a b s t r a c t

Kaolin is used in many applications (e.g. paint, plastic and rubber) due to its unique properties, includingfine particle size, brightness and chemical inertness. Properties can be improved by calcination, heatingthe kaolin to high temperatures inducing physico-chemical transformations. If calcination time is exces-sive, kaolin becomes harder and abrasive, due to mullite formation. This is detrimental to product quality;causing processing problems and machinery damage. If calcination time is too short, the kaolin is not fullycalcined, becoming highly reactive, an issue for pharmaceutical applications. By controlling the exposuretime to high temperature, such negative effects are minimised. Multiple hearth calciners are often used

esidence timeultiple hearth furnace

alc tracer

for this transformation, but their operating conditions and configuration make it difficult to determinekaolin residence time. To improve product consistency, trials were performed to measure the residencetime distribution of an industrial furnace used to calcine kaolin. Titania, TiO2, and talc, Mg3Si4O10(OH)2,were used to individually dope the kaolin, Al2Si2O5(OH)4, and the concentration of these mineral tracerswas detected using X-ray fluorescence. Talc was the preferred mineral tracer, with a mean residence time

ope −1

of 42 min under standardfeed.

. Introduction

Kaolin, often known as china clay, is principally made up of theineral kaolinite, which has the formula Al2Si2O5(OH)4. Calcina-

ion, the process of heating a substance to a point below its meltingr fusing point, causing a loss of water from the structure, is one ofhe most important ways of enhancing the properties, and value, ofaolin. On calcination, the kaolin becomes whiter and more chemi-ally inert, allowing it to be used in a wide variety of products, suchs paper, rubber, paint and refractory items.

When kaolinite is calcined, it under goes a series of physico-hemical reactions [1,2]. The initial reaction occurs around00–150 ◦C and involves all adsorbed moisture being driven off.etween 400 and 600 ◦C the kaolinite undergoes a dehydroxyla-ion reaction, the removal of chemically bonded water, to produce

etakaolin, which also has industrial applications [3,4]. Dehydrox-lation causes a change in the co-ordination of the aluminium fromix- to four-fold. The aluminium–oxygen tetrahedra then becomes

stretched-out’ over the unaltered silicon–oxygen network [3].

The calcination of kaolin at around 980 ◦C leads to the trans-ormation of the metakaolin to the spinel phase by exothermice-crystallisation [5]. The exact reaction which takes place is sub-

∗ Corresponding author at: Imerys Minerals Limited, Par Moor Centre, Par Mooroad, Par, Cornwall PL24 2SQ, UK. Tel.: +44 1726 818198; fax: +44 1726 818015.

E-mail address: rachel.thomas@imerys.com (R. Thomas).

255-2701/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.cep.2009.01.003

rating conditions, with a throughput of 5.3 tonnes h of 0 wt.% moisture

© 2009 Elsevier B.V. All rights reserved.

ject to much debate and the only generally accepted theory is thata spinel-type phase forms along with some free silica, although thetype, its chemical formula and the mechanism of formation, are allstill under discussion [3]. Spinel is a collective term for a group ofoxides which have the general formula X2+Y2

3+O4 (where X and Yare universal chemical symbols). However, due to aluminium defi-ciencies in the metakaolin structure, vacancies appear in the spinelwhich are integral to its structure and so the term defect-spinel isused [5]. Within the defect-spinel, the aluminium has reverted toits octahedral co-ordinated state [3]. Above this temperature, thekaolin begins to form mullite, which is hard and abrasive and cancause damage to process equipment, if undetected.

The many different techniques for the calcination of kaolin canbe grouped into three distinct types. Soak calcination is the tradi-tional method; and involves exposing kaolin for extended periodsof time, often up to 24 h, to high temperatures. The resultant prod-uct is hard and abrasive, primarily composed of mullite. This formof calcined kaolin is, generally, used in refractory applications, suchas kiln furniture or investment casting.

The second type involves stopping the calcination reactionbefore significant mullite formation; the product is considered tobe ‘soft calcined’, as the abrasive properties are minimized, but

the product is whiter and brighter than the hydrous form whilstalso being chemically inert. This can be achieved by a variety ofmethods, although principally in industry three different calcinersare used: a rotary calciner, a fluidised bed calciner, or a multiplehearth calciner. All have much smaller residence time than the

ring and Processing 48 (2009) 950–954 951

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Table 1Showing the X-ray fluorescence results for kaolin, talc and titania (with resultspresented as percentage by weight of the elemental oxides).

Kaolin (wt.%) Talc (wt.%) Titania (wt.%)

Al2O3 38.6 4.7 3*

SiO2 56.9 65.9 5*

K2O 2.77 <.01 0Fe2O3 0.73 0.09 0TiO2 0.02 0.01 92*

CaO 0.07 0.29 0MgO 0.27 28.66 0Na2O 0.13 <.01 0

of the blades on the rabble arms can be varied in order to change theresidence time within the kiln. Burner settings can also be adjustedin order to give a specific temperature profile over the kiln. In thisparticular kiln, heating to specific preset conditions is provided withnatural gas burners on the 4th and 6th hearths; these are controlled

Table 2Showing MgO content of various calcined talc/kaolin blends determined by X-rayfluorescence.

Talc/kaolin ratio MgO content (wt.%)

0/100 0.281/99 0.413/97 0.74

R. Thomas et al. / Chemical Enginee

oak methods and use material agitation to improve heat transferniformity and, thus, product consistency.

Flash calcination is a more recent development. It involveseating the kaolin very rapidly, usually to between 900 and000 ◦C within a fraction of a second, achieving heating rates of00,000–500,000 ◦C s−1. This is achieved by dispersing the kaolinarticles within a hot air stream, ensuring a good rate of heat trans-

er. The steam evolved from the transformation of the kaolin cannotscape from the individual particles, causing the formation of sealedlisters, adding a novel functionality. The sealed air blisters give

mproved light scattering effects in paints and plastics and the den-ity of calcined kaolins is significantly reduced due to entrapped air.his reduces the quantity needed in applications to give the sameffect as normal mineral fillers, giving manufacturers a worthwhileost saving [6,7].

. Experimental

.1. XRF pressed powder technique

Samples taken from the kiln were examined for compositionalariation using the XRF pressed powder technique. This gives ahemical analysis of various kaolins, giving results as percentagesf the different oxides present. For each analysis, a mixture of 4 g ofample and 1 g of polyvinyl alcohol (PVA) powder was weighed intocontainer and shaken in a mixer mill for 5 min. The fully dispersedixture was then placed in the well of a motorised laboratory pellet

ress and made into a pressed disc with a diameter of 32 mm.Before each operation, the XRF spectrometer was calibrated

sing a suite of standards of pressed powder. The discs were runn the XRF spectrometer and the fluorescent intensities for eachlement recorded. Using the recorded intensities and the knownhemical composition of the standards, the calibration parametersor each element can be calculated by the operating software.

.2. Powder flow testing

The powder flow properties of each of the powders involved inhis study were carried out using a Freeman FT3 Powder Rheome-er (Freeman Technology, Malvern, UK). Samples were prepared by

easuring 160 ml of powder into the sample vessel and the massequired was recorded. A conditioning cycle was then performedefore any testing was started [8].

The Freeman FT3 Powder Rheometer measures the Basic Flowa-ility Energy (BFE), the Stability Index (SI) and Flow Rate Index (FRI).he BFE test consists of a standard conditioning cycle followed by aest cycle with a downward transverse tip speed set at 100 mm s−1

nd a 10◦ negative helix. This produces a compaction effect at aigh flow rate. The SI is measured by repeating the BFE measure-ent seven times on the same sample, with a conditioning cycle

etween each test. The value for the 7th test is then divided by therst to give the SI. The FRI test consists of four testing cycles with aownward 10◦ negative helix at tip speeds of 100/70/40/10 mm s−1

ith a conditioning cycle between each test. The BFE at 10 mm s−1

s then divided by the BFE at 100 mm s−1 to give a ratio [8].

.3. Tracer selection

Initially, titania, TiO2, and talc, Mg3Si4O10(OH)2, were both cho-en as potential mineral tracers, as both titanium and magnesiumre not normally found in abundance in South West UK kaolins.

his paucity allowed samples to be analysed, by X-ray fluorescenceXRF), to detect any deviation of these elements, Mg or Ti, fromormal levels, so allowing the reaction time distribution to be deter-ined. Table 1 shows the XRF composition data of talc, kaolin and

itania.

LOI 0.5 0.5 0

* Values for titania are estimated from industry known values as pure titania isdifficult to measure using XRF techniques.

Table 1 shows that there is some MgO present in the kaolinused for the trials. It was, therefore, necessary to perform somecalibration experiments so that the amount of talc present in theproduct could be determined. Samples with different talc/kaolinratios were calcined, under controlled laboratory conditions, andthe product examined using XRF, in order to ensure that the min-eral tracers did not have a detrimental effect on the calcined kaolinproduct, and that it could still be detected after heating. The resultsare shown in Table 2. The particle size of the two tracer particlesused is of a similar distribution to that of the feed to the calcinerwith a 300 mesh size of less than 0.02 wt.% and with a 2 �m contentof 83 wt.%.

2.4. Industrial calciner configuration and operation

The majority of the soft calcined kaolin is produced using con-tinuous multiple hearth calciner technology. The multiple hearthcalciner equipment examined in this research consisted of eighthearths. The kiln is circular, 6.8 m in diameter, each hearth is 1.1 mhigh and has a diameter of 6.1 m. The distance between hearths is0.93 m and the central column diameter is 0.87 m.

The multiple hearth kilns are configured so that the feed kaolinis fed into the top of the kiln and is then pushed towards a centraldrop hole by rabble arms, which are essentially a series of motorisedrakes. On the second hearth, the kaolin moves from the inside of thekiln to the outside and falls through drop holes in the outside of thehearth [9] as shown in Fig. 1. The kaolin continues to fall alternatelyon the inside and the outside of the hearth as it progresses throughthe kiln.

Under standard operating conditions, air flow to the kiln is8000 N m3 h−1, whilst the feed rate to the kiln is approximately5.3 tonnes h−1 of 0 wt.% moisture feed, although this may differ,depending on the feed quality. The rotational speed and orientation

5/95 2.1310/90 3.7720/80 8.0950/50 16.01100/0 28.66

952 R. Thomas et al. / Chemical Engineering a

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of titania rich material to pass through into the product withoutbeing sampled. The total quantity of TiO2 detected in samples fromthe kiln is around 33.6% of the total added to the kiln. It is unknownhow long the remainder took to exit the kiln, as it was not identifiedby the sampling protocol used.

ig. 1. Diagrammatic representation of a multiple hearth calciner with eight hearthsetailing major internal components of flows of powder material.

o meet product specifications, the maximum gas temperature theiln is operated at is 1100 ◦C.

The exact temperature profile of the kaolin as it moves throughhe kiln is not known. This is due to a number of factors that makesampling impossible: the kiln is in continuous operation; it is oper-ting at extremely high temperatures and several moving internalarts are present. As such, detail on the residence time distributionf individual particles is not known and this can have significantmpact on the performance of the product. Estimates can be madeor the duration inside the kiln using geometry, feed rates and esti-

ates of density changes. However, it is impossible to say howccurate these are.

.5. Tracer addition

Kaolin is added to the top of the kiln in a semi-continuous man-er using a loss-in weight feeder. Powdered kaolin feed is metered

rom the feed storage bin with the feeder into a discharge augernd through a rotary airlock valve into the kiln. The feeder at theop of the calciner was adapted to incorporate a small hopper, whichould be isolated by means of a slide valve, situated directly overhe discharge end of the screw. The hopper was loaded with 100 kgf tracer. The tracer powder was then discharged quickly into theuger and timing started. This configuration allowed the entirety ofhe mineral tracer to be dosed into the kiln in one go.

.6. Sampling

Calcined kaolin is discharged from the kiln through two dropoles on either side of the kiln base, from which the calcined pow-er falls through a discharge auger. This material is then movednd dropped into an air blast cooler, where the discharged cal-ined kaolin is both cooled and transported to a dust collector, fromhence it is sent for milling to deagglomerate the particles.

Samples of the calcined material were taken from the exit ofhe kiln as it dropped from the kiln discharge point and before it

assed into the air blast cooler. As the material being discharged

s still at high temperature, samples were taken with an insulatedteel long handled device which was passed across the powder bedn a sweeping motion to obtain an even, characteristic sample ofroduct. Approximately 100 g of sample were taken each time.

nd Processing 48 (2009) 950–954

In the initial titania experiment, both sides of the product streamwere sampled, to determine whether there was any preferentialflow through to one particular side of the kiln. Samples were takenfrom side B every 5 min for the first 15 min, and then every minutethereafter until 60 min had elapsed. Further samples were thentaken every 5 min until 75 min had passed. Side A was sampledevery 5 min throughout the operation.

Once it had been determined that there was very little differ-ence between the two sides as a result of the titania experiment, itwas deemed not necessary to sample both sides for the talc trial.Samples were taken as the feed kaolin was dosed with talc, 5 minlater and then every minute until 59 min had passed. Sampling thenrestarted at 80 min and samples taken every 5 min until 170 min.One final sample was taken at 180 min after the original dosingtook place.

3. Results and discussion

3.1. Titania tracer experiments

As shown in Fig. 2, titania, TiO2, begins to appear in the productat around 18–20 min. The level of TiO2 then rises to a plateau, with acouple of recorded exceptions at 34 and 51 min where the TiO2 lev-els reach 4.6 and 7.5 wt.% respectively. Once the plateau of around0.4 wt.% TiO2 is reached, it does not descend during the period ofthe trial.

Simple mathematical modelling involving the flow rate of thefeed and the distance through the kiln, suggests that the residencetime within the multiple hearth kiln should be just below 40 min, ifplug flow is assumed. This experiment, however, indicates that plugflow does not occur and that there is a great deal of short circuitingand recirculation. In spite of this, there is a high level of internalmixing occurring within the kiln, as both sides of the kiln showsimilar results.

The lack of TiO2 in the first 20 min of sampling indicates thattitanium dioxide did not flow straight through the kiln, and wasmixed in with the kaolin. The two large spikes that occur at 34and 51 min giving values of 4.6 and 7.4 wt.% respectively, however,seem to show that some of the TiO2 may have travelled down thekiln in fairly condensed clumps of material. If this is the case, itmeans that any mass balance results would be very unreliable asdue to the method sampling, it could be possible for large portions

Fig. 2. Variation of the titania, TiO2, content of the calcined product sample duringthe period immediately following the titania tracer dosing addition.

R. Thomas et al. / Chemical Engineering and Processing 48 (2009) 950–954 953

Table 3Base flow energy, stability index, flow rate index and mass data for talc, calcinedkaolin and titanium.

Base flow energy(mJ)

Stability index Flow rate index Mass (g)

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alc 1019 0.952 2.29 121aolin 799 1.02 1.76 72itania 3274 1.07 1.33 127.4alcined kaolin 799 1.02 1.76

The difference in specific gravity for the two materials is thoughto be the cause for some of the inaccuracies. Kaolin and calcinedaolin have specific gravity values (relative to water) of 2.6 and.65, respectively, whilst TiO2 has a much higher value of 4.2. It is,herefore, believed that the titania was not carried along with theulk of the kaolin inside the kiln, and instead, travelled differentlyhrough the kiln; possibly, with some segregation from the kaolinr agglomeration. This difference in flow properties is reinforced byhe Freeman Rheometer data for the materials, as shown in Table 3.

A perfect flowing powder would have a flow rate index of 1, as allour powders have a flow rate index of higher than one, this showshat all of them are sensitive to a change in flow rate, with talcnd hydrous kaolin being slightly more responsive than the otherwo minerals investigated. The stability index seems to show thatll four powders are reasonably stable, as all four minerals have aalue very close to 1, which identifies a completely stable powder.

However, the titania has a BFE of more than four times that ofalcined kaolin, three times that of talc and eleven times greaterhan hydrous kaolin. This indicates that a greater amount of energys required to move the titania than the standard kiln product oreed. As this is the most important value for determining how theowder will react in an industrial situation, it can be seen that TiO2ows in a different manner to kaolin and is therefore not suitableo be used as a tracer for this operation and the results shown inig. 2 are believed to be a poor representation of the residence timeor kaolin in the kiln.

Since kaolin and talc had more comparable flow properties, ashown by the almost identical flow rate index values and similarpecific gravity values exhibit by the two minerals, talc was chosens the tracer. The difference in stability index is considered minimalnd, although, the base flow energy for the three minerals differs,t is hoped that this will not cause problems in the kiln.

.2. Talc tracer experiments

The results for the talc experiment are shown in Fig. 3. The talcegins to arrive at the outlet after about 20 min. Some talc, how-

ig. 3. Graph showing change in the talc content (inferred by MgO content) of thealcined product during the talc tracer dosing experiment.

Fig. 4. The amount of soluble aluminium detected in calcined kaolin exposed to atemperature of 1050 ◦C for varying duration.

ever, remains in the kiln for up to 80 min. By the time 90 min haveelapsed, all of the dosed feed has passed through the kiln and thetalc recovery is total. The peak amount of talc in the feed is at 43 min,when it is calculated that 4.2 wt.% of the product is talc. The meantime for a particle to spend in the kiln is determined to be 42 min,which equates to around 5.25 min per hearth.

The initial prediction, based on feed rates and known kiln geom-etry, was that the residence time would be just below 40 min. Thedifference between the measured value and the theoretical modelvalue may be due to the way the feed moves through the kiln andsubtle changes in kiln geometry due to thermal expansion. Thesteady peak shown in Fig. 3 indicates that the flow through thekiln is dominated by a plug flow regime, as expected. However, thelarge variation in residence time indicates that the flow of powderis not entirely uniform and that the feed follows a more complex,random route through the kiln.

This finding does have some serious implications for the productand its consistency. If the kaolin passes through the kiln in 20 min,it is highly unlikely that it has been calcined sufficiently. As shownin Fig. 4, a sample that has only been exposed to temperature for20 min, under laboratory conditions, has a high level of soluble alu-minium present in it; indicating that the calcined product has somereactivity associated with it. This is due to the calcination reactionnot being completed and the sample being made up of metakaolin.Once the spinel phase is reached, the amount of soluble aluminiumpresent in the product decreases dramatically. This can be seenin Fig. 4 to be occurring at around 30 min. At the same time, thebrightness level (as measured by light reflectance at a wavelengthof 471 nm) is increased, as shown in Fig. 5.

Therefore, any portion of the kaolin that is not properly calcined

may have a detrimental effect on the product, reducing brightnessand increasing the reactivity. Any kaolin that is held in the kiln for anextended period will also have an adverse effect. It will not affectthe easily identifiable characteristics of calcined kaolin, a kaolin

Fig. 5. The brightness of calcined kaolin exposed to temperature for varying dura-tion.

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hich is calcined for around an hour, shows very little increasen brightness, as can be seen in Fig. 5, and the amount of solubleluminium, remains stable, Fig. 4. The problem is that the kaolinill have been more thoroughly calcined than expected and may

ontain mullite. Even small amounts mullite may cause damage toachinery or increase equipment wear rates, which is undesirable.Adjustment of the rabble arm blade configuration, the manip-

lation of the feed addition profile to avoid slug dosing, andncreasing the number of feed entry points to the kiln could all be

anipulated to alter the residence times. Each method is known toinimise any hold-up or accumulation in the kiln, and reduce the

egree of short circuiting, creating a more uniform product.The wide distribution of possible kiln residence time may cause

ifficulties when the system is operating outside of normal condi-ions, for example during trials and results may not be as expected.herefore, if long-term changes were to be made to the kiln, such asf a different product were to be made or if the feed material wereo change, it is recommended that further tracer experiments bearried out.

. Conclusions

Under standard operating conditions, with a throughput of.3 tonnes h−1 of 0 wt.% moisture feed, the mean residence time

nside the multiple hearth kiln has been determined to be a meanf 42 min with the use of talc as a tracer. Talc was discovered toepresent the flow properties of kaolin more accurately than tita-ia which is understood to move at a different rate and by a differentechanism through the kiln to kaolin. This is probably due to dis-

repancies in the powder flow behaviour, as determined with thereeman Powder Rheometer, and also the significant difference inpecific gravity between the kaolin/calcined kaolin and the titania.

The range in residence time of dosed kaolin in the kiln is overn hour, which could have serious implications for the quality and

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nd Processing 48 (2009) 950–954

consistency of the product. Kaolin that has only been in the kiln fora short period of time will not be fully calcined and if it has reachedthe metakaolin stage will be reactive with poor colour; propertiesin stark contrast to that desirable in calcined kaolin.

The similarity between the two sides of the kiln shows thatthere is a good degree of mixing in the kiln, however, if significantchanges were to be made to the kiln, such as operating conditionsor feed material, it is recommended that further tracer trials takeplace.

Acknowledgements

The authors would like to thank Imerys Minerals Ltd. and,through the Engineering Doctorate (EngD) scheme, the EPSRC fortheir support and funding for this project.

References

1] C.S. Ross, P.F. Kerr, The kaolin minerals US Geological Survey Pro Pap 165E, 1931,pp. 151–180.

2] R.E. Grim, Clay Mineralogy, McGraw-Hill International Series on the Earth Plan-etary Sciences, McGraw-Hill, New York, 1968.

3] L.T. Drzal, J.P. Rynd, T. Fort, Effects of calcination on the surface properties ofkaolinite, J. Colloid Interface Sci. 93 (1) (1983) 126–139.

4] E. Gamiz, M. Melgosa, M. Sanchez-Maranon, J.M. Martin-Garci, R. Delgado, Rela-tionships between chemico-mineralogical composition and color properties inselected natural and calcined Spanish kaolins, Appl. Clay Sci. (2005) 269–282.

5] H. Schneider, K. Oka, J.A. Pak, Mullite and Mullite Ceramics, John Wiley and Sons,Chichester, England, 1994, pp. 105–145 (Chapter 4).

6] C. Agra-Gutierrez, D. Whiteman, High Performance Fillers 2006: 2nd Interna-tional Conference on Fillers for Polymers, Proceedings of a Conference held atCologne, Germany, 21–22 March 2006, Shawbury, Rapra Technology Ltd., 2006,Paper 3, p. 8.

7] D. Stewart, Quality Comparisons Extenders and Fillers, Editorial, PPCJ, June 2008,pp. 8–22.

8] B. Powlesland, Evaluation of the FT3 Powder Rheometer, Imerys Internal Memo,2003.

9] D. Grose, The enhancement of flash calcine clay by secondary calcination, MScthesis, 2004.