lime slaking

8/8/2019 Lime Slaking

1/6

1

Slaking of lime

Johan B. HolmbergDepartment of Chemical Engineering II, Lund Institute of Technology,P. O. Box 124, S-221 00 Lund, Sweden

Quicklime is produced when limestone is heated above 900 C, a temperature at which

limestone decomposes to carbon dioxid and quicklime (CaO). Quicklime exists in manydifferent qualities. Limes of different quality have been characterised, wet slaked and dry

slaked. The characterisation in combination with the wet slaking suggested that therelationship between the surface concentration of calcium and carbon was the best measure

for the lime reactivity in the slaking process. The higher value for the relationship the morereactive was the lime. The wet slaking tests showed that the differences in reactivitydecreases when temperature is raised or calcium chloride is added to the water. The dry

slaking tests showed upon very small differences considering the slaking degrees for

different conditions and different qualities of lime. However the tests showed on arelationship between water content in the reactor and the specific surface for the product,the higher quote the higher specific surface.

Introduction

Quicklime is produced by heatinglimestone above 900 C, a temperature at

which limestone decomposes to carbondioxid and quicklime (CaO). The productquality is among other things dependent on

this calcining process. Prolonged heatingor heating at too high temperatures

generate a less reactive lime. Anotherproblem is how the calcining process isperformed, what sort of furnace that is

used. Besides how the calcining process isdone, is the product quality governed by

the amount of impurities in the lime.Quicklime is often used in many

processes involving slaking, it is therefor

of utmost importance knowing whether the

quicklime will be slaked or not in theslaking process. To test this quicklimes ofdifferent qualities were investigated. Thelimes tested were a couple of Chinese

limes, a Swedish lime and a lime fromPoland.

Characterisation of quicklime

The normal measurements whenquicklime is characterised are the amount

of free lime and total lime in the product,

the porosity of the lime, the amount ofimpurities and the particle size distribution.

Only three different limes were fully testedand characterised. Six additional Chineselimes were tested for impurities, the lime

content and reactivity. The three limes thatwere fully tested were they from Poland,

Sweden, a lime from Partek Nordsjkalk,and one from China

The tests for available lime showed on a

distinct difference between the limes.

Table 1 Content available lime

Type of lime Available content (%)Swedish 91Chinese 78

Polish 66

To test the porosity a whole varity ofvariables were tested since it was notpossible to measure the porosity directly.

The measurement made were the specificsurface, the pore size distribution, the pore

volumes and the density.

Table 2 Specific surface (BET)

Type of lime BET-surface (m2/g)Swedish 1.15Chinese 2.67

Polish 1.10


2/6

2

Table 3 Average pore size (BJH)

Type of lime Pore diameter ()Swedish 144Chinese 200Polish 182

Table 4 Pore volumes (BJH)

Type of lime Pore volume (cm3/g)Swedish 0.0035Chinese 0.0118

Polish 0.0044

Table 5 Density

Type of lime Density (kg/m3

)Swedish 1210

Chinese 1250Polish 1300

Since quicklime belongs to the bulkchemicals it is difficult to measure the

density. The density is therefor given asthe figure for the weight per volume when

the lime no longer could be compressedthrough shaking.

The figures listed gave no real answersto how reactive each lime would becompared to the others. The density

implies that the Swedish lime should bemuch more reactive than both the polish aswell as the Chinese lime. The measured

specific surface and the measured porevolumes indicate that the Chinese lime

should be much more reactive than boththe Swedish and the Polish lime, whichshould show almost the same reactivity.

The average pore size for the different limegave no new light to which lime that is the

most reactive.

Table 6 Particle size distribution

Swedish Chinese Polish


3/6

3

temperature rise was measured and thiswas correlated to the conversion. The

slaking procedure was carried outaccording to the ASTM-standard C110.Besides that, the initial temperature

dependence was tested.

The Swedish lime at different

starttemperatures

0

10

20

30

40

50

0 5 10

Time (min)

Tempdiff(C

25 degreas

30 degreas

35 degreas

40 degreas

50 degreas

The Polish lime at different

starttemperatures

0

5

10

15

20

25

30

0 10 20 30

Time (min)

Tempdiff(C

25 degreas

30 degreas

35 degreas

40 degreas

50 degreas

The Chineese lime at different

starttemperatures

0

10

20

30

40

0 5 10Time (min)

Tempdiff(C

25 degreas

30 degreas

35 degreas

40 degreas

50 degreas

Figure 1 Different limes at different starttemperatures

In the figure above we can see bigdifferences in slaking times for the Polishlime to the others and we can also notice

that the Chinese lime is a little morereactive than the Swedish lime. We canalso notice the slaking time seems to

converge with raised start temperature forthe slaking. Parts of the fact that the Polish

curve increases its reactivity depends onthe fact that the time difference betweentotal slaking time and the slaking time

according to the ASTM-standard levels outwith raised start temperature.

Another way of raising reactivity is byadding calcium chloride.

Table 8 Slaking with addition of CaCl2additive none CaCl2

Type of lime Time (min) Time (min)Swedish 6.33 2.67

Chinese 4.67 2Polish 17 7.67

The figures in the table above are theslaking times you get if you add 10 grams

of calciumchloride per litres of water. Thefigures clearly show that the slaking timesdecreases fairly when you add

calciumchloride.

Figure 2 The Polish lime wet slaked with calcium

chloride

Whats interesting in the graph above is

that there is a minimum slaking time whencalcium chloride is added and that the lessreactive the lime is the more effect has

increasing the dose of calcium chloride.

Limes slaked differently

0

20

40

60

0 20 40Time (min)

Tempdiff(C)

Polish lime

Polish lime

plus 20 g

CaCl2x2H2O

Swedish lime

Polish lime

50 C

Figure 3 The Polish lime compared to the Swedish

Polish lime with different ammonts of

calciumchloride added

0

10

20

30

40

0 10 20 30

Time (min)

Tempdiff(C) 0 g/ l

4 g/ l

10 g/ l

20 g/ l


4/6

4

What you can see in the last figure isthat when both raising the start temperature

and adding calcium chloride to the slakingwater the difference in slaking timesbetween the Swedish lime and the Polish

lime decreases dramatically.As with many positive effects there is

also a downside. Adding calcium chloridelowers the BET-surface of the slakedproduct, see Table 9.

Table 9 BET-surfaces

Additive none CaCl2Sort of lime Surface

(m2/g)Surface(m2/g)

Swedish 26.5

Chinese 23.4 9.7Polish 18.2 7.9

The surfaces in the table above are those

for the products slaked at a startingtemperature of 25 C.

The third way of raising the limesreactivity is grinding. The Polish lime was

ground, the fraction above 125 m was

ground to under 125 m, when you

compare this reactivity to that one for theoriginal lime you notice that reactivity has

increased dramatically, as you can see inthe figure below

Polish lime ground

010

20

30

40

0 10 20 30

Time (min)

Tempdiff(C)

Polish lime

Polish lime

ground

Figure 3 The Polish lime ground

The six additional samples from Chinawere also slaked. These samples were

slaked with another equipment totallyaccording to the ASTM-standard. Since

these latter samples were slaked with thesame equipment and at the same time they

were fully comparable. Besides one of thesamples all samples had a well definable

slaking time according to the ASTM-standard. However one of the sampleslacked this cause there were no

temperature measurement made in the endbesides the end point. Plotting the slaking

times against the ratio between calciumand carbon atomic surface concentrationsgave an almost perfect linear function apart

from the outlier discussed earlier.

The slaking time versus the

relationship between calcium and

carbon on the surface

0

5

10

1520

25

0 0.2 0.4 0.6 0.8 1

Ca/C (atomic concentrations)

Slakingti

me

(min)

Figure 4 The slaking time plotted against surface

concentrations of calcium and carbon

The slaking seems to be divided into twoparts one initial part that probably involves

bursting of the particles and a second phasethat seems to be a part where the reaction

goes on, on the surface of the cores of theparticles. The data from the wet slakingwere fitted to the shrinking core model for

a surface reaction.

Shrinking core reaction control for the

Swedish lime

0

0,2

0,4

0,6

0,8

0 100 200 300

Time (s)

1-(1-x)^1/3

25 degreas

30 degreas

35 degreas

40 degreas

50 degreas

Figure 5 Shrinking core reaction control for the Swedish

lime

The model was valid for conversions

between 0.4 and 0.9. The figures wereadjusted for the different temperatures and


5/6

5

the temperature dependence was calculatedaccording to the arrenhius equation and

gave the activation energies for the slakingprocesses, 38 kJ/mol for the Swedish lime,56 kJ/mol for the Polish and 60 kJ/mol for

the Chinese lime.

Dry slaking

The limes from Sweden and Poland

were also slaked in a two-step dry slaker.The slaker could be modelled as a tank

followed by a tube. What was measuredwas the specific-surface and slakingdegrees of the slaked products. The slaking

degrees were measured with a TGA, where

the slaking degrees could be calculatedfrom the weight loss when the lime washeated above 600 C, a temperature atwhich lime decomposes into quicklime and

water.It was showed that the slaking degrees

for the Polish lime were close to those forthe Swedish lime. The slaking degrees are

correlated to the content available lime.

Polish lime

0

5

10

15

20

25

30

0 0,2 0,4 0,6 0,8

Relationship lime/water (kg/kg)

BET-surface(m2/g)

5.85 kg/h

4.12 kg/h

5.2 kg/h

Figure 6 The specific surface plotted against the water

dosation

In the figure above you can clearly seehow the specific surface of the slaked

product depends on the water content inreactor. This result was not dependent ofthe lime quality.

Slaking with addition of Triethanolaminwas tested for the Swdeish lime. The major

difference was that BET-surface doubledand that the amount of small pores in theproduct increased.

Results and disscussion

It was showed that the only really goodway of measure and characterise lime is by

measuring the relationship between

calcium and carbon on the surfaces. Thecorrelation between the reactivity and thedifferent surface concentrations maydepend on some sort of mass transport

problem associated with the amount ofcarbon on the surfaces or it may somehow

be connected to the fact that there is muchcarbonates in the lime.

It has further been shown that the wet

slaking times for different limes can bedecreased by raising the temperature or by

adding calcium chloride or by doing both.This temperature dependence behaviourmay explain why there is only small

differences between slaking degrees in thedry slaking.

Its also suggested that the differences inspecific surfaces in the dry slakingexperiments arise from the fact that the

temperature is lowered when extra water isadded and that this might have an positive

influence on the specific surface.

Literature cited

ASTM C 110-87 (1987) Standard Test M ethods frPhysical Testing of Quick lime, H ydrated L ime andL imestone

Babatschev, G.,Kassabova, M. (1969) E influss vonTemperatur und E lek trolyten auf die H ydration von

ungelschtem Kalk, Zement-Kalk-Gips (22)312-316

Becker, H., Zander, Von H. (1976) Uber dieN eutralisationsgeschwindigkeit von nass oder trock en

Swedish lime

0

10

20

30

40

0 0,2 0,4 0,6 0,8Relationship lime/water

(kg/kg)

BET-surface(m2/

g)

5.2 kg/h

6.7 kg/h

7.8 kg/h

3.7 kg/h


6/6

6

gelschten Kalk hydraten in A bhngigk eit von derenH erstellungsbedingungen, Zement-Kalk-Gips (29)381-387

Becker, H., Zander. Von H. (1977) Uber dieL sungsgeschwindigk eit von Kalk hydraten, Zement-

Kalk-Gips (30) 287-292Boynton, R. S. (1966) Chemistry and technology of

lime and limestone, 1:a upplagan, John Wiley &Sons

Campbell, I. M. (1988) Catalysis at surfaces, 1:aupplagan, Chapman and Hall

Devismes, von R., Foster, P., Perraud, R. (1990)E ntschwefelung von schwefeldioxidreichen rauchgasenmit branntkalk und kalk hydrat, Zement-Kalk-Gips (43) 38-42

Frank, G., Achenbach, G. (1987) E influss vonChlorid-Ionen auf das L schen von Kalk, Zement-Kalk-Gips International (40) 479-482

Frank, G. (1977) E influss der L schbedingungen aufdie Qualitt des gebildeten Calciumhydroxids beim

N asslschen von Kalk, Zement-Kalk-Gips (30)34-39

Giles, D. E., Ritchie, I. M., Bing-An, X. (1993)The k inetics of dissolution of slaked lime,Hydrometallurgy (32) 119-128

Schmitz, F., Hennecke, H. P., Bestek, H.,

Roeder, A. (1984) Trock engelschtes Kalk hydratmit grosser Oberflche-E in wirk sames reagenz zur

bindung saurer abgasbestandteile Teil1: Herstellung

im labormastab und ausblick ber die verwendungbei der rauchgasreinigung, Zement-Kalk-GipsInternational (37) 530-533

Hennecke, H. P., Kning, W., Roeder, A.,Schmitz, F., Stumpf, T. (1986) Trock engelschtesKalk hydrat mit grosser Oberflche-E in wirk sames

reagenz zur bindung saurer abgasbestandteile Teil2: A ufbau und betrieb der k leinproduktionsanlage;

betriebsergebnisse aus versuchen z ur trock ensorptionvon schadstoffen aus unterschiedlichen abgasen,Zement-Kalk-Gips International (39) 251-258

Ingesson, E (2000)H ydrox id OH- sock ermetoden

Johnson, W. A., Mehl, R. F. (1939) Trans AIME(135) 416

Levenspiel, O. (1972) Chemical reaction engineering,2nd upplagan, Wiley international editions

Ohnemller, W., Hupe, B. (1969) D ie H ydrationdes Branntk alkes in der Kalk sandstein-Rohmasse

und ihre Bedetung fr die Steinfestigk eit vor und nachdem Dampfhrten, Zement-Kalk-Gips (22) 116-121

Ritchie, I. M., Bing-An, X. (1990) The kinetics oflime slak ing, Hydrometallurgy (23) 377-396

Schlegel, E., Werner, W., Hartmann, H-J. (1976) Z ur H ydrationsgeschwinigk eit von CaO,Silikattechnik (27) 377-378

Zeilnhofer, J., Ploetz, C. (1998) V ollautomatischeKalk -Trock enlschanlage, Zement-Kalk-GipsInternational (51) 494-499

R eceived for review february 5, 2001

lime slaking

Documents