MAINTENANCE

lmproved mud-washingand flocculents bringringing under control

BY H. TRAN, J. GRIFFITHS,AND ItI. BUDGE

NVIRONMENTAL REGULATIONS ONmalodorous gas emissions frompulp mills have become stricter in

recent years. As a result, lime kilns areincreasingly used to burn and destroynon-condensible gases (NCG) and totalreduced-sulphur compounds (TRS) from

kiln ringing problems

recovery processes. The practice has longbeen suspected as a source of ringingproblems in lime kilns []. Industry-wideexperience, however, has varied.

Not all NCG-burning kilns have ex-perienced severe ringing problems. A spe-cial meeting, Ringformation in lime kilnsburning NCG, was held in January of1988 by the National Council of Pulp andPaper for Air and Stream Improvementin Atlanta. There was considerable debateamong the 67 participants from 19 Ameri-can pulp mills as to whether NCG com-bustion is the prime cause of ringing.

This paper describes the experience ofringing problems, and the measures takento minimize them at E.B. Eddy ForestProducts' IlOG adtld bleached kraft pulpmill in Espanola, Ontario.

The Espanola millThe Espanola mill has two identical

Allis-Chalmers lime kilns, No.l andNo.2. The kilns were started up in 1965

and 1983 respectively. Each kiln isequipped with a product cooler, has anouter dimension of 3.05 m x 76 m and aproduction capacity of 160 t/d.

The lime mud feed, which comes froma common single-l ine Dorr-Oliverrecausticizing system, is washed on twopre-coat drum filters operating in parallelahead of each kiln, Fig. l. The No.l kilnis fired solely with natural gas. The No.2kiln can burn either natural gas or bunkerC and in the future will be able to burntall oil.

As part of the mill's environmentalprotection program, NCG, includingsteam stripper vapors (methanol, turpen-tine, etc.) and TRS have been burned inboth kilns.

Up to March 1987 the No.l kiln,which burned most of the NCG and TRS,experienced severe ringing problems. Thisgreatly hindered its production capacityand imposed a greater load on the alreadyundersized kiln drive mechanism. The

Experience of lime

at E.B. Rldy Forest Products

L

TRAN

BUDGE GRIFFITHS

AA H. Tran, Pulp and Paper Centre,116$ University of Toronto, Toronto,

Ontario; J. Griffiths, M. Budge,E.B. Eddy Forest Products Ltd., Espa-nola, Ontario.

#2 Pr€cost #l Pr€co8tFilter Filter

#l PrmostFilter

R.bldLlm Sb

R.burrdUe Sk

Fig. 1. Common lime feed to kilns.

T26PULP & PAPER CANADA 92:1 (1991)

MAINTENANCE

isn3

nI

DIIIrII

It,

ring consistently formed at about 18 mfrom the hot end of the kiln. where therefractory changes from 0.23 to 0.15-mthick, Fig. 2. The ring occasionally grewto 0.6 to 0.9-m thick and was up to 9-mlong at its base. Chemical analysis of theNo.l kiln ring deposits showed that therings contained on average about l09osulphur by weight(wt9o), primarily calci-um sulphate (CaSOo). The sulphur con-tent was usually much higher at thelocations where heavy rings were ob-served.

In the No.2 kiln, where NCG and TRSwere only occasionally burned, rings alsoformed at the same location as in theNo.l kiln, but they were much thinnerand softer. The severe ring formationproblem in the No.l kiln was thus at-tributed to its frequent combustion ofsulphur-containing NCG and TRS.

The investigationKiln shell tempenture profiles: Three kilnshell temperature profiles for the No.lkiln were measured using an infraredpyrometer to examine the location of ringformation during operation. The meas-urements were taken five,22 and 33 daysafter the kiln clean-out on March 5, 1987.As the ring built up, it insulated the kilnshell from the burner flame, causing theshell temperature to decrease,

In spite of the use of thinner refracto-ries at l8 m from the hot end of the kiln,a decrease in kiln shell temperature overtime was observed, suggesting the forma-tion of rings at this location, Fig. 3. Theresults coincided with the visual inspec-tion during the kiln shutdown.

The kiln shell temperature peaked atabout 12 to 15 m from the hot end. in-dicating that this was the location wherethe temperature was highest in the No.lkiln. Rings did not form at this location.Ring deposit analysis: Samples of ringdeposits were collected from three to 47m from the hot end of the No.l kiln andanalyzed for Na, S and Ca through neu-tron activation analysis using the Univer-sity of Toronto's Slowpoke reactor.Figure 4 shows the sodium content in thering deposits. Both water soluble and in-soluble sodium content is expressed as theNa/Ca mass ratio as a function of dis-tance from the hot end of the kiln.

The results were scattered, but they didshow a decrease in total sodium contentin ring deposits at locations closer to thehot end. The low Na content in the ringdeposits close to the hot end is presuma-bly due to the vaporization of sodiumcompounds from ring deposits at hightemperatures.

The sulphur content in the ringdeposits was quite different. The S,zCamass ratio was about 0.2 at the hot end,

PULP & PAPER CANADA 92:1 (1991)

Reburned l ime

Fig. 2. Location of rings in the kiln.

IiIIIItIt i1il r

3l!(rl

TEIrJo-

ut

JJLllI(t,

zjY

,f OO

350

300

250

200

1 5 0

100

5020 30 40 50 60DISTANCE FROM HOT END (m)

-X- MAR 9/87--F MAR 1ol07

* MAR 27/67

Fig. 3. No.1 Lime kiln shell temperature profiles.

0.030

0.025IF

ff o.ozoFIg 0.015IIJ=I 0.010(!

z0.005

0.00010 20 30 40 50 60

DISTANCE FROM HOT END (m)

70 80

Fig. 4. Sodium content in ring deposits.

T27

MAINTENANCEincreased markedly to about 0.7 at l8 m,and then decreased steadily to 0.15 atabout 45 m from the hot end, Fig. 5.

In the region where the S./Ca ratiopeaked (20 to 30 m from the hot end) thering was hard and fused, as shown byscanning electron microscopy (SEM), Fig.6. The results suggest the key role of sul-phur in ring hardening.Water-soluble Na in lime mud: Thewater-soluble Na content in the lime mudfed to both kilns was measured. Up toMarch 1987, the average monthly water-soluble Na content in the lime mud fedto the No.l ki ln was about 0.15 wt9oNatO, compared to only 0.08 wt9o NarOin the lime mud in the No.2 kiln, Fig.7.

The difference was caused by the oper-ation of only one of the two precoat mudfilters ahead of the No.l kiln, a practicethat was deemed necessary at the time toprevent over-drying of the lime mud. Ifthis was not done, the drive motor for thekiln feed screw might have tripped-out.

Because of an undersized drive unit.the No.l kiln mud feed was limited toabout 4090 of the total mud fired to bothkilns. This smaller quantity of mud couldhave over-dried when passed over the twoprecoat filters ahead of the No.l kiln.

The higher water-soluble Na contentin the lime mud fed to the No.1 kiln gaverise to speculation that sodium in the limemud may also have contributed to thering formation. Sodium compounds inthe mud react with SO2, SO3 and CO. inthe flue gas to form NarSOo and/orNarCOr. These would melt at about820"C, the temperature that prevails inthe calcining zone. This might cause dustparticles to adhere to the refractory sur-face to form rings.

The so-formed rings consist mostly ofCaO and are generally soft. They cannotgrow because of the abrasive action of therotating and sliding motion of reburnedlime pellets. With the combustion ofsulphur-containing NCG and TRS,however, the already formed rings reactwith SO, and SO, to form CaSOn.

CaO + SO, + l/2 02 = CaSO+

C a O + S O 3 = C a S O ,

The dust particles are thus chemicallybound together, forming hard rings whichare resistant to the abrasive action of lirnepellets. Such "sulphated" rings wouldthen be able to grow thicker over time.

Sulphation reaction is less thermody-namically favorable at high temperaturesand low SO, or SO, atmosphere. In a fluegas containing 100 parts per million (ppm)SO3, for instance, CaSOn would not format temperatures above 1200"C, or woulddecompose to CaO and SO2 or SO, if it

80

Fig. 6. SEM photograph of a ring deposit at 20 m trom the hot end.

habil

lortetR(thselmdta(ct

ul

alNf(dtit(ti

tldN

,:i Ii;\IIII

IT

0.800

0.700

o o.oookf o.soo-t-

6 o.+oouJ3 o.soo(UoD o.2oo

0.100

0.00010 20 30 40 50 60

DISTANCE FROM HOT END (m)70 80

Fig. 5. Sulphur content in ring deposits.

o.20

o .1 6

o . 1 2

o.08

0.04

o.o0

I r r l rue xr tx N *z r - rue xr lx

of

(Eozoa'oN(t2

86-2ND 87-1ST 87-2ND 88-1ST 88-2ND

Fig. 7. Water-soluble sodium in lime mud to No.1 and No.2 lime kilns.

T28PULP & PAPER CANADA 92:1 (1991)

MAINTENANCEhad already formed. The thermal insta-bility of CaSOo was probably the mainreason that rings were not observed at thelocation nearest the burner, where thetemperature was the highest.Remedial measures: Speculation aboutthe effect of sodium led to the return toservice in April 1987 ofthe second precoatmud filter prior to the No.l kiln. Thedrive motor problem was overcome byadding a small amount of water to the dis-charge screw of the second filter.

With both filters operating, the sodi-um content in the No.l kiln mud droppedalmost 5090, from 0.15 to 0.08 wt9oNa2O, Fig. 7. Since then, the rate of r ingformation in the No.l ki ln has alsodecreased, enabling the kiln to run con-tinuously from one scheduled shutdownto another. This has greatly decreasedlime requirements and labor costs forcleanouts, as well as the need to landfillthe lime mud during unscheduled shut-downs when the mill is still operating.

Since 1988, the sulphur input to theNo.l kiln has also been reduced by burn-ing more NCG/TRS in the No.2 kiln. Thewater-soluble sodium in lime mud to bothkilns has increased, Fig. 7. This ispresumably because of the higher mudload to both kilns from the increased pulpproduction.

There was no clean-out attributable toringing in 1988. However, this was prob-ably because of the increased number ofshutdowns of both kilns: l0 for No.l andseven for No.2. These shutdowns wereeither scheduled or caused by breakdownsin kilns and/or other mill areas. In bothcases, rings would break off.

Figure 8 shows the monthly average ofthe mill white-liquor sulphidity sinceI 986. Because of increased tall oil produc-tion, the sulphidity increased from 34 to3890 on active alkali from April to De-cember 1987. No severe ringing problemswere experienced in either kiln during thishigh-sulphidity and low-sodium period.This suggests that sulphur input alonecannot account for the ringing problem,particularly in lime kiln No.l.

The cause of ringingExperience suggests that ringing

problems in the No.l kiln were caused bythe combination of high-sulphur andhigh-sodium inputs to the kiln. The high-sulphur input resulted from the combus-tion of NCG and TRS, while the high-sodium content in lime mud may increasethe stickiness of dust particles, enhancesintering through liquid-solid diffusion atthe particle joint, and accelerate the sul-phation reaction via a molten phase.

The reactions between sulphur-bearinggases, CaO and CaCO, to form CaSO,would make the already-formed rings

PULP & PAPER CANADA 92:1 (1991)

ffi',*.$FF*Fig. 9. Deposit built-up in slurry line header.

Fig. 10. Deposit built-up an slurry line header after water washing.

d

w' #

' , i ,

!i ii' d l

:.,$rls

$$

T29

i

IIIII

38

37

3..t r s so

o-= soJl( ' ) 3 3

32

3 1

3087-1ST 88-1ST 88-2NO

Fig. 8. Monthly average white liquor sulphidity (on active alkali).

MAINTENANCEhard and difficult to remove. The effectof sulphuration on ring hardening and therole of sodium in ring formation are be-ing investigated at the University ofToronto.

Clogged scrubber linesBefore the second precoat mud filter

was put back in service the scrubber slurrylines would need cleaning at about thesame time as the No.l kiln was scheduledfor a shutdown or a ring cleanout. Thelonger operating time of the No.l kiln fol-lowing the restart thus caused problemsof its own. The slurry recirculation linesof the No.I kiln venturi scrubber becameclogged with water-insoluble CaSOn-CaCO, scale. The rock-hard scale built uprapidly on slurry line surfaces, restrictingthe flow to the scrubber, the mud mixbox, and the bleach plant sewer. It alsooccasionally broke off and plugged pumpsuction.

To clean the slurry lines, the No.l kilnhad to be shut down for three days whilethe scrubber pipe work was dismantledand cleaned with high-pressure water. Thecost of the cleanout was about $33 000.

The clogging problem in the slurryrecirculation line has been overcome byadding Limefloc, a Dearborn flocculentthat prevents the formation of hardscales. When it is added to the scrubberrecirculation lines the deposit still ac-cumulates in the slurry header, but it issoft and clayJike, Fig. 9. As it builds, theeffective diameter of the header isdecreased causing the flow velocity to in-crease. This, in turn, helps to reduce thedeposit formation rate, maintaining thedeposit at an equilibrium thickness.

Although this clay-like deposit in theheader may be massive, it can be easilycleaned down to bare metal in a fewminutes with a simple water hose, Fig. 10.Little deposit was found to accumulate inslurry pipe lines where high-flow veloci-ty prevails.

Flocculent use appears to make theCaSOo-CaCO, deposit loose, bulky andeasy to remove. Since its use, the No.lkiln and its scrubber lines have operatedfrom one scheduled shutdown to another.

ConclusionsThe Espanola mill experience suggests

that ringing problems in the No.l kilnwere caused by the combination of highsulphur and sodium inputs to the kiln.

The high sulphur input resulted from thecombustion of NCG and TRS, while thehigh sodium input was caused by poormud washing.

Both the ringing and cloggingproblems in the No.l kiln have beenbrought under control by the restartingof a precoat filter to improve mud wash-ing, and by the use of Limefloc.

AcknowledgementsE. Cuadra, University of Toronto isthanked for assistance with the chemicalanalysis.

ReferencesL BURGESS, T.L., 1989 Kraft Recovery OperationsSeminar, 145, TAPPI (1989).

R6SUm6: Cette communication d6crit I'exp6rience de la formation de dep6ts dans les fours d chauxet de I'encrasssement des conduits de recirculation des boues liquid€s chez Produits Foresliers E.B.Eddy. Ces problemes semblent reli6s d la combustion de gaz non condensables, et empires par lecontenu 6lev6 en sodium dans les boues r6siduaires de chaux. On constate que I'ajout d'un filtreprimaire pour am6liorer le lavage des boues r6siduaires de chaux ainsi que le lait de minimiser I'apporten soufre dans le four ont diminud consid6rablement la formation de d6p6ts. Le probldme de I'encras-sement a 6t6 a I'utilisation d'un floculeux.

Abstract: The paper describes the experience of ringing in lime kilns and of clogging in slurryrecirculation lines at E.B. Eddy Forest Products Ltd. The problems appear to be related to thecombustion of non-condensible gases, and aggravated by the high sodium content in the limemud. By restarting a precoat filter to improve lime mud washing, and minimizing the sulphurinput to the kiln, the rate of ring formation has been significantly decreased. The clogging problemhas been overcome by the use of a flocculent.

Reference: TRAN, H., GRIFFITHS, J., BUDGE, M. Experience of lime kiln ringing problemsat E.B. Eddy Forest Products. Pulp Paper Can 92(l): T26-30 (Jan. l99l). Paper presented atthe 1989 International Chemical Recovery Conference of the Technical Section, CPPA, co-sponsored with TAPPI at Ottawa, on April 3 to 6, 1989. Not to be reproduced without permis-sion. Manuscript received March 3, 1988. Approved by the Review Panel, October l'1 ,1989.

Keywords: LIME KILNS, RINcs, FUELS, DEposITIoN, ANALYSIS, SoDIUM coM-POUNDS. MAINTENANCE. THEORIES. SULPHUR COMPOUNDS. POLLUTIONCONTROL.

T30

INDUSTRY NEWSLAYOFFS

Domtar cuts 300white-collar jobsMONTREAL, Que. - In response to theworst nine-month financial performancein Domtar Inc.'s history, 300 white-collarworkers from its head and division officeshave been laid off. President and CEOPierre Desjardins announced the job cutsas part of the overall restructuring pro-gram initiated in the fall of 1989. The pro-gram also included a similar layoff of 750office employees a year ago, and the saleof over $140 million worth of non-strategic assets. In explaining how Dom-tar will manage with 1000 fewer heads,Jean-Daniel Hamelin, communicationsmanager, told PULP & PAPER CANA-DA that the decentralization drive beeun

82

last year will continue. Responsibilitiesand decision making have been shifteddown to the division and, in some cases,the plant level.

stitution's "American Papermaking: 300years" exhibition. Information can be ob-tained from Martin Koepenick at404-984-2260.

ANNIVERSARY CLOSURES

Celebrating 300 yearsof U.S. papermakingATLANTA, GA - Vision 300, a non-profit, non-political group of industryprofessionals has been formed tocelebrate 300 years of American paper-making. Vision 300's objective is topreserve a vision of the 300-year historyof American papermaking, and to createa vision for the future. The group willprovide funding through corporate spon-sorships for anniversary celebrations,school contests. and the Smithsonian In-

CPFP to closeToronto box plantTORONTO, Ont. - Canadian PacificForest Products Ltd. plans to shut its cor-rugated container plant in Rexdale, Ont.,laying off over 100 workers. Roger H6-bert, vice-president, administration, saidthe plant was no longer economical, andwill be shut to reduce manufacturing costsand improve customer service. The mill'sproduction will be consolidated at thecompany's five remaining corrugatedplants.

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PULP & PAPER CANADA 92:1 (1991)

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