Burnability And Clinkerization Of Cement Raw-mixes

BURNABILITY AND CLINKERIZATION OF CEMENT RAW-MIXES T.K. Chatterjee Mysore Cements Limited UC0 Bank Building, Parliament Street New Delhi 110 001, India

1. INTRODUCTION

T

he chemical, physical and mineralogical behaviour of raw-mix considerably influences its burnability and reactivity. This ultimately explains clinker formation in the presence of solid liquid gaseous environments through extremely complex physico-chemical transformations at regular temperature intervals. The characterization and evaluation of rawmix is essential for achieving suitabfe design of the raw-mix, smooth operation of the kiln and cooler and better quality of clinker. The reaction sequence during raw-mix sintering covers both solid- and liquid-phase mechanisms, with the formation and decomposition of regular intermediate compounds which, in turn, get transformed into major clinker phases, such as C3S, &C2S, C3A and C,AF. This paper attempts to highlight the burnability and reactivity of raw-mix and its effects on the behaviour of raw materials, as well as the reaction sequence and kinetics of clinker formation, with a view to ascertaining their importance in a clinker-making process. A study on these lines is obviously difficult but will, however, provide positive guidelines for further development in the existing technology, reduction in energy consumption, improvement in clinker quality and optimization in system design.

2. BURNABILITY Burnability of raw-mix has been a matter of great importance in cement technology. Raw-mix behaviour during its sintering process is greatly influenced by its chemical, mineralogical and granulometric compositions. Variations in these affect kiln operation, refractory lining, fuel consumption

Burnability and Clinkerization

I1

and clinker quality. Each cement raw-mix burns in its own way, resulting in variation of clinker quality.

2.1 Definition The burnability of a cement raw-mix conceptually denotes the amount of mass transfer of its constituents with ease or difficulty to the clinker phases. By convention, burnability is measured by determining the CaO, (free lime) after burning the raw-mix for a certain time (19) at a certain temperature (T); i.e. CaO, = f (0, T) when melt is formed, above 1300°C burnability decreases by increasing this parameter.

2.2 Expression Burnability is generally expressed by either of the following two quantities: - Measure of CaO, of a pseudo-isochrone (0= const.) at a given temperature. Increasing values of CaO, correspond to decreasing burnability. - Measure of time (0) of a pseudo-isotherm (T = const.) for CaO,<2%. The increasing of 8 corresponds to decreasing burnability.

2.3 Factors Affecting Burnability The following are the important parameters which considerably affect the bumability of raw-mix: 2.3.1 Raw-mix-Mineralogical composition - Lime components - Clay components - Corrective ingredients - Modifiers 2.3.2 Raw-mix-Chemical composition - Main component relations - Minor non-volatile components - Minor volatile components 2.3.3 Raw-mix-Granulometric composition - Fineness - Particle-size distributions - Homogeneity and compaction 2.3.4 Raw-mix-Thermal Treatment - Firing temperature - Heating rate - Burning period - Burning activation

12 Progress in Cement and Concrete 2.3.5 Liquid phase formation - Appearance temperature - Amount - Viscosity - Surface tension - Ionic mobility 2.3.6 Clinker quality - Silicate phases - Alumino-ferrite phases 2.3.7 Coal ash

- Amount absorbed - Composition - Fineness 2.3.8 Kiln atmosphere - Oxidation - Reduction 2.3. I Raw-mix-Mneralogical Composition Cement raw-mix represents a polymineral and polydispersive mixture whose composition can vary within a wide range due to the character of raw materials used. In the clinker-making process, 90% of the raw-mix constituents comprise the four major oxides, viz. C, A, S and F, and the remaining 10% is made up of minor constituents. These oxides occur in the form of minerals and compounds in the raw materials and dissociate into oxides through high-temperature treatment in kiln. Constituents and compositions of raw-mix are shown in Table 1. Table 1. Raw Mix-Constituents and Compositions Raw Mix

Corrective Lime components

Clay components

Consisting mainly Consisting mainly of CaC03 and a of SiO2 with very small quantuy considerable of the following in amounts of the the order: following in the S-M-R-F-S-N-K order: R-F-C-M-S-N-K

ingredients

Modifiers

Consisting mainly of any of the main Consisting of oxides (C,A,S,F) different inorganic compounds which accelerate the clinkerization reactions.

Burnability and Clinkerization 13 2.3. 1. 1 CaO-carriers

Calcite, aragnonite, dolomite, ankerite, etc. are the main carriers of CaO. The dynamics of carbonates in the raw-mix depend on the type of carbonates, their crystal structures, microstructural peculiarities, and the dispersability of crystals and impurities present. The dissociation temperature of the individual mineral and appearance of CaO in the most reactive state decreases in the order: Calcite (aragonite)-dolomite-ankeritecl) 2.3. 1.2 Acidic oxides IS, A and 17 carriers

It is well known that SiOZ and A1z03 are found in raw-mix in the form of various clay minerals (kaolinite, montmorillonite, hydromicas, chlorite, etc.) micas, amphiboles, epidote, pyrophyllite and feldspar; Fez03 and Al203 are often found in diaspore, bohemite, hydrohaematite, hydrogoethite, goethite, etc. The reactivity of clay minerals with CaCOJ increases according to the following order: muscovite-montmorillonite-chlorite-illite-kaolinite~~~~~ Amorphous Si02 or Si02 combined with A&O3 and/or CaO, and/or Fez03 shows better reactivity than free SiOZ. The reaction of different forms of silica with CaO increases in the following order: quartz-chalcedony-opal-cr-crustobalite-o-tridymite-silica of feldspar+silica of micas and amphiboles-silica of clay minerals-silica of glassy slags(r) Minor volatile and non-volatile components are always present in main oxides. The concentration of minor constituents in the raw-mix is in the following order(h): M - K - S - N - Ti - Mn - P - Sr - F - a - Cr The temperature and rate of volatilization of miner volatile compounds are dependent on the mineral form of materials bearing these components. As, for example, S from pyrites volatalizes at much lower temperature than from gypsum. Similarly, alkali from silica vaporizes at lower temperature than feldspars. 2.3. 7.3

Correc rive ingredients

Whenever necessary, the raw meal is corrected with small amounts of ingredients to adjust the raw-mix design in the desired range. Usually siliceous materials, laterites, bauxite, pyrite, flue dust or sand (5) are used, depending on the lacking oxide and the material available at the lowest cost. 2.3.1.4

Modifiers

These are minor components called fluxes and mineralizers acting in the liquid and/or liquid-solid phase. Whereas fluxes lower the temperature at which the liquid phase appears, mineralizers accelerate the rate of clinkerization@. CaFz, Na$SiF6, CaSO,. 2H20, Caj (PO,),, etc., are often used in a very small quantity with the raw-mix as modifiers.

14

Progress in Cement and Concrete

2.3.2 Raw-mix-Chemical Composition Raw-mix constitutes four main oxides (C. A, S and F), together with the minor volatiles (K, N, S, p, F, fi and H and the non-volatiles (Sr, M, Ti, Mn, Sr and Cr). Each component of the raw-mix has individual and combined (M,, MA, LSF and Ms) effects on its bumability, which has been illustrated in Table 2. The approximate range of chemical composition of raw-mix and the potential clinker minerals formed after burning the rawmix are shown in Table 3. 2.3.3 Raw-mix-Granulometric Composition 2.3.3. 1 Fineness

and

particle-size

distributions

Fineness and particle-size distributions greatly affect raw-mix burnability. The more fine grained the raw-mix with greater surface area, the easier it is to sinter and lower the sintering termperature. In some raw-mixes, further grinding has particularly no influence on burnability. Increasing the coarseness of alumina and limestone is found to have an effect on raw-mix burnability whereas the coarseness of quartz is found to have a marked effect(43). One percent quartz grains over 100 pm can be regarded as equivalent to 6% calcite grains of the same sizecU). Increasing the size of Si particles from 0.09-0.15 mm to 0.3-0.46 mm increases CaOr from 0.5 to 0.8% at 1550°C but increasing the coarse silica from 0.5 to 2.0 mm and burning at 1500°C for 30 min. increases CaOr from 0.7 to 3.7%f45). So it could be pointed out that more than 0.5% Si particles about 0.2 mm size of 1% between 0.09 and 0.2 mm should not be present in the raw-mix. Calcite particles above 0.15 mm size can be tolerated without serious effect, while with enriched siliceous limestone, the proportion of coarse particles could be higher. In fact, the maximum permissible particle size of quartz, feldspars and calcite is recommended to be 44 pm, 63 pm and 125 pm, respectively@@. The normal target of fineness of raw-mix is 12% residue on 170 BSS mesh and 2.6% residue on 72 BSS mesh. This target differs from plant to plant. Control of particle size is very important since the sintering rate is roughly proportional to the inverse of the particle sizd4’). It has also been seen that the reactivity of cement raw-mix decreases linearly with the reciprocal value of the product of the squares of the average calcite and quartz grains(4@. 2.3.3.2 Homogeneity and compaction

Homogenization of kiln feed is a major operation in cement manufacturing, as it affects the quality of clinker, burning process and fuel consumption. Low reactive materials like quartz, due to their differential grinding, concentrate primarily in the coarse fractions of raw-mix and thus disturb their homogeneity. It is very difficult to get an absolutely homogenized mix since there are always grain contacts between A-A, A-S, S-S, etc., resulting in micro inhomogeneity. Mineral homogeneity is increased by using the least number of raw materials in a mix.

Burnability and Clinkerization 15 ?A

Parameter/ Characteristic 3. Limesaturation factor (LSF)

Empirical formula

L S F , = - loo’ 2.8S+ 1.65A+0.356 (MA >0.64) 1OOC LSFz = 2.8S+l.lA+0.7F W,t Q0.W LSF = _ lOO(C +0.75M) 3 2.8S+ 1.18A+0.6+ (Mg2.0) lOO(C+UM) or = 2.8S+ 1.8A+0.65F(M > 2.0)

B. Minor Non-volatile Components 1. Free silica Sl (SiO, in different forms)

Limiting range

Preferable Effects range

Ref.

0.66 - 1.02 0.92 - 0.96 A higher LSF 1. makes it difficult to burn raw-mix 2. tends to produce unsound cement (high CaO,) 3. increases C$i content 4. reduces C2S content 5. reduces C,S content 6. causes slow setting with high early strength

O-3%

as low as possible

A higher St 1. increases power and fuel consumption 2. causes difficulty in coating formation 3. deteriorates refractory lining 4. increases the radiation of heat from kiln shell 5. increases kiln exit-gas temperature

8,

12-15

003 3 2 se Q 3 3 : 5 s R a 3

8

(Table 2 Contd.)

Parameter/ Characteristic

Empirical formula

Limiting range

Preferable range

2. hlagnesium oxide (h&O)

M

O-S%

o-2%

3. Titanium oxide (TiO,)

O-4%

1S-2%

Effects

’ A Higher M 1. reduces viscosity and surface tension of clinker liquid and increases ionic mobility 2. favours the dissolution of C,S and CaOl at higher temperature and lets C,S form more quickly. 3. tends to ball easily in the burning zone which affects kiln operation 4. leads to unsoundness by forming periclase crystals when M > 2% 5. increases CpS and melt but has no effect on C2S (M <2Qo) 6. volume instability in the presence of 2 )M 26, neutralizes when Qo SO, = 0.67 A higher Ti 1. results in sharp reduction in C,S content with equal gain in C# content and appreciable variations in other phases. 2. reduces viscosity and surface tension of the melt 3. reduces grain sizes of alite and belite 4. causes slower setting and lower early strength 5. forms darker colour clinker

Ref.

18

Progress in Cement and Concrete

(Table 2 Contd.) Parameter/ Characteristic

Empirical formula

Limiting range

Preferable range

Effects

Ref.

C. Minor Volatile Components 1. Alkalies (KsO + NazO)

(K+N)

o-l%

0.243%

A higher (K+N)

l&u),24 29-30

1. improves burnability at lower temperature and deteriorates at higher temperature, specially when (K + N) > 1% 2. increases liquid content and coating formation 3. lowers the solubilty of CaO in the melt 4. breaks down alite and belite phases 5. creates operational problems due to external and internal alkali-cycle formation (volatility (K>N)+ 6. when (N% + 0.659 Kilo) >0.6%-causes alkali expansion 2. Sulphur compounds (SZ-, sop, sot-

s

O-4%

OS-2%

A higher S 1. acts as an effective mineralizer and modifier of the alkali-recycle by forming less volatile (N,K) SO4 compounds&en S > (K + N)+ 2. lowers the appearance temperature of liquid phase by oved lOO”C, decreases its vtscostty and surface tension and increases ionic mobility of oxides.

(Table 2 Contd.) Parameter/ CllUWthtiC

8

Empirical formula

Limiting range

Preferable range

Effects

Ref.

2 8 ii

3. increases belite formation where there is no effect on alite or melt 4. decomposes alite at 1250°C if high alkali sulphates are present. 5. when SO, >2.5-4.0%, causes sulphate expansion 6. improves burning of raw-mix at lower temperatures and deteriorates the same at higher temperatures 7. decreases hydraulic and mechanical strength 3. Phosphorous pentoxide (pm

P

O-l%

0.3-0.970

A higher P 1. accelerates the clinkcrixation reaction 2. reduces the intensity of internal recycle 3. reduces early strength 4. reduces C3S content

4. Fluoride (PI-

F

O-0.6%

0.03-0.08%

A higher F 1. leads to hrgher values of Pco, = f(t) and modifies the kinetics of all the burning reactions 2. lowers the jemperature of C,S formation by 1 so-200°C 3. has no effect on the internal cycle in the kiln 4. decreases mechanical strength of the clinker

(Table 2 Contdl Parameter/ Characteristic

Empirical formula

Limiting range

5. Chloride (Cl’ -)

O-0.6%

6. Moisture OH’-)

lo-35%

Preferable range

Ref

Effects

O-0.015%

A higher Cl 1. forms more volatime (K,N) Cl and causes operational problem due to its complete vaporization in the burning zone 2. increases liquid formation and melting point of the absorbed phase is drastically changed 3. increases ring formation by readily forming spurrite (2CrS.CaCOs) 4. a by-pass is required if ni>O.O15

15% (dry)

A higher H 1. increases bumability 2. increases fuel consumption loss on ignition

30-35%

Wet)

When M,, = 1.23. liquid appears at 1338T in C-A-S-F system. MA = 1.63, liquid appears at 1301°C under MgO saturation conditions. t when g(N +K) < 1.25 -alkali bypass is not required(*) # Sulphate modulus, M&42) is defined as S/O.85 (K + 1.52N) Whsn Ms < 0.5-S bounds to 0.5 >MQ< 1.0 - an increasing portion of S bounds to calcium (CaSO,), Mi = 1.0 - a constant fraction (70-90%) of the alkalies found as sulphate, (N,K)$$04.

l

alkalies,

8,20,35

22 Progress in Cement and Concrete Table 3. Composition of Raw-Mix and CUnker Minerals Raw-mix-Composition Chemical composition LOI C s A F M W+N S

Clinker minerals-Composition

Chemical Mineralogical Range (o/o) composition* Range (Vo) composition+ Range (@IO) 35*2 42*2 14*2 5i2 2fl 3kl.5 0.7 f 0.3 l.Szt1.2

LOI C S A F M W+N) S

0.3 f 0.2 62&S 20*6 6i2 3&2 3.5+ 1.5 0.7sto.3 1.5k1.2

c,s c2s C3A

C,AF M CaOr

55*10 25+10 9*4 11*4 3.5* 1.5 l+l

Traces of other minor constituents. LOI = Loss on ignition + Liquid - 24 f4%

l

fluctuation of the kiln feed measured as 070 CaCOj should not be more than k0.29’0 from the holding point. An increase of 1% CaC03 will increase CgS by 13% and reduce C2S by about 11.5%. The ultimate homogeneity depends on the physicochemical characteristics, fineness and particle-size distribution, method of mixing and efficiency of the blending system. 2.3.4 Raw-mix Thermal Treatmtnt 2.3.4.1 Firing temperature

In clinker burning, the temperature must be fairly enough for the formation of the alite phase. Burning of raw-mix is generally carried out at 14%1500°C. An excessivelv high burning temperature results in high stress on the kii and the refractory lining, more fuel consumption, reduction in cement strength(* and larger alite crystalso@. Increase in burning temperature from 1360 to 1420°C results in lowering the burning period by half. Maximum firing temperature was determined by a multiple regression analysi@) of raw meal containing only the four main oxides as given below: “C = 1300 + 4.51 C3S - 3.74 C3A - 12.64 C&F (1) 2.3.4.2 Holding time

On increasing the holding time, the following changes may be observed(“): 1. C3A content decreases and C&F content increases. 2. Q3 decreases and C3S increases. 3. Higher mechanical strength at later ages and lower at early ages. 4. Heat of hydration at early ages decreases. 5. Unburnt clinker produces highquality cement even in the presence of high CaOr.

Burnability and Clinkerization

23

2.3.4.3 Burning rate

Rapid burning is favoured for the following reasons(52$53): 1 . More coarse-grained materials can be charged 2 . Materials differing in their degree of fineness can be charged. 3 . Fine grains of C# are formed which accelerate the interaction of CzS, CaOr and liquid. 2.3.4.4 Burnmg activation

Thermal activation may be enhanced by either accompanying it with mechanical (vibratory mill) or chemical (mineralizer) activation. Mechanical activation gives better results than chemical activatio@J. 2.3.5 Liquid phase of formation A, F, M-minor volatile and non-volatile components-generally govern the amount of liquid formed, its appearance, temperature, viscosity, surface tension and ionic mobility in the clinkerization process, which is explained in Table 2. The range of clinker composition may be fairly wide if the amount of liquid phase increases slowlyC~s). A clinker with about 25% liquid phase from a raw-mix is generally considered an ideal raw-mix for kiln lining, fuel saving, rapid C3S formation through the dissolution of CIS and CaO, and economical clinker grinding. The liquid phase at 1450°C is usually calculated by the Lea and Parker formulacss) which is written as: 3.0 A-2.28F+K+N+M (2) when MA > 1.38 (2) 8.5 A-5.22F+K+N+M (3) when M, c 1.38 (3) 2.3.6 Clinker Qua&y It has been see@) that the burnability becomes worse as the potential C3S content increases at the expense of other clinker constituents, while an increasing C3A and C&F potential content improves the burnability the C&F being significantly more effective in this respect. 2.3.7 Coal Ash Ztt~ence When coal is used as the fuel for clinker-making, its ash content, composition and fineness affect bumability. Generally the composition of coal ash varies within the limits: S-35-60%, A-15-35%, F-5-20%, C-O-IO% and M, ?l and alkalies are often present in the ash in small amounts. In general, the ash composition shows a very high S/C ratio and moderately high A/F ratio. On the whole, the effects of ash absorption on burning are as follows: 1 . LSF decreases and Ms increases. 2 . The composition of silicate phases changes. 3. Liquid content increases with reduced viscosity and increased ionic mobility. 4 . Molten ash penetrates rapidly from the outer surface into cracks and openings in the clinker ininerals by permitting the ash to react in pockets, resulting in C,S-rich areas(m. 5 . Introduction of a degree of microinhomogeneity due to ash-clinker reaction, resulting in some reduction in the strength.

24 Progrcrs in Cbnent and Concrete 2.3.8 Kiln Atmosphere Reducing conditions (inadequate oxygen in the kiln gas) during cement clinker-burning substantiahy affect the colour of the clinker by producing ferrous oxide, accelerate the setting by enhancing C,A content at the expense of C,AF, and reduce the strength by breaking down C,S during clinker coolir@~. Therefore, oxidizing conditions (O-l-2 vol. % in exit gas) should be maintained in the kiln for better clinker quality.

3. RAW-MIX CHARACTERIZATION AND EVOLUTION The characterization and evaluation of raw-mix is essentially done through some tests for optimizing the burning of clinker raw meal. These tests are accompanied with two divisions: (A) Proposed raw-mix (B) Laboratory-prepared clinker

(A) Proposed Raw-mix A raw-mix is prepared similar to that proposed to be used to manufacure cement. The raw-mix should then be evaluated by the following routine analysis: 1. Fineness and particle-size distributions 2. Chemical analysis 3. X-ray diffraction 4. Volatility test 5. Differential thermal analya. 6. Thermogravimetric analysis 7. Burnability test

(B)

Laboratory Prepared-clinker

Laboratory prepared-clinker is made by burning the proposed raw-mix for a certain time (usually 60 min) at 1400°C. The laboratory prepared clinker is characterized by: 1. X-ray diffraction 2. Scanning electron microscope In this chapter only the techniques for the evaluation of raw-mix bumabiity will be discussed. The burnability of raw-mix can be characterized by: (i) a purely theoretical approach, (ii) a semi-experimental approach, or (iii) a purely experimental approach.

3.1 Theoretical Approach In this approach, empirical equations are used for calculating the bumability index (BI) factor (BF) through physical or chemical character istics of rawmix. The empirical relations shown in Table 4 are derived by different authors.

Burnability and Clinker&ion

25

Tab&k 4. Empirical Equations Used for Burnabi&y In&x Ga&&tion Burnability index/factor

Empirical

BI, BIZ BFI

C3S/C,AF + C,A C3S/C,AF+C,A+M+K+N LSF+IOMs-3(M+K+N) LSF+6(Ms-2)-(M+K+N) 55.5+ 11.9R++ + 1.58 (LSF, - 9oy - 0.43 Lf

BFz Btll

equation

Equation Reference No. (4) (5)

58 59

(6)

60

(7)

59 61

U-9

Where L, = amount of liquid phase at 1350°C after IMII@~ R +9o*m = mass % raw meal retaines on 9Ow sieve.

The BF seems to be more practical as it involves LSF and moduli. In the case of BI, the potential composition of phases are calculated, using Bogue’s equation(43). However, Bth, which takes into account the chemical characteristics, granulometry, heterogeneity and liquid content, seems to be more accurate.

3.2 Semi-experimental Approach In this approach, a minimum number of experiments are used in empirical equations to calculate the burnability indices.

3.2.1 Freei%ne Temperature Integration iUethodc61, In this method, the burnability capacity is measured by a quantitative expression related to the CaOr = (‘I,@ trend in the overall 10W’C to 1450°C range, maintained for 20 min, at each temperature and is expressed by: B C =

y

(9)

where C is the sum of total CaOr at different temperatures (lOOO-145O”C), expressed as : +4c MOOT + ~%o”c

(10)

This method seems to be rational, as it evaluates bumability on the basis of the course of reactions in the overall lOOO-145O’C range.

3.2.2 Statistical MO&W In this method, ten parameters have been chosen to determine burnability indices. These are MS, LSF. M& R+~o,,,,,, (N+ K), M, S, S+ZOO,, Ms.MA, F and mica minerals (GI). The model expressed by: CaOis’ 1400°C =

0.022Y - 1 - J(O.022Y - 1)s + 0.01 l&W 0.008Y

26 Progms in Cbnent and Concrete Y = Y,f + g aiXi

(12)

i=l

and Y&= .

CaOref - 5 - 0.oo4@0,f - 5>2 + O.O22(CaO,,r

- 5) + 1.174 (13)

where CaOrd xi

a,

= CaOf of the reference raw-mix = difference between value of each parameter and reference mix, = (constant), coefficients of xi defining the regression.

3.2.3 Chemico Granulometdc Approachc6J, In this approach, both chemical and granulometric composition are taken into account and CaOr is determined affter firing at 1400°C for 30 min. Caq’,,., = 0.33 LSFs + 0.018Ms + 0.56CaCOs. izrm + 0.9353+,, - 0.349 (14) This equation is justified for moderate variations from the reference sample, representing the average values: LSF, = 0.953 f 0.038; (Jaw + 12sw = 0.026 f 0.021 S +o)cm = 0.018*0.020 MS = 3.7*1.5; CaOt = 0.057 f 0.043 MA = 2.2 f 1.6 and However, for larger variation in LSFs and Ms, a non-linear expression will be needed to fit data. The above approach was further modified recently@@ by the following expression: Caq’ r,., = 0.33 (LSF-(LSF) (MS)) +0.93S+44cm + 0.56 CaCO, + lurm + 0.24 (15) where LSF (Ma) is a function of Ms and determined experimentally by a correlation analysis: LSF (Ms) = -5.1 Ms+ 107 (16) This equation is valid for 88
3.3 Experimenti Approach The best way to determine the bumability of a raw-mix is by an experiment In the hboratory which takes care of all known and unknown factors hfhmchg It. ‘i’be C&f content in the burnt clinker thus becomes the single

Burnability and Clinker&ion

27

criterion to arrive at any conclusion. Several methods are derived for measuring CaOf experimentally.

3.3.1 Bumability Scalef~~ BS =

070 CaOf (10% LOI)

% cao,

(17)

One gram of raw-mix is heated at 1100°C for 20 min. and then the CaOf is determined. The LO1 and CaOf are determined from the original raw-mix. A higher BS signifies a harder burning mix.

3.3.2 Practical Bumability@l) In this method, Practical Burnability, I& is determined by the time required for firing the raw-mix in a rotary furnace at a constant temperature of 1350°C to attain a CaOf <. 2%. This results in the possibility of the prediction of a theoretical burnability, Bc,...e.q. 8 (correlation coefficient, 7 = 0.94) The temperature-dependent practical burnability may be derived@@ from the CaOf - 8 slope and follows an exponential equation: B*r = B1sa exp (0.0126 (1360-T)) WI In another approach, CaOf -8 was further modified by accounting simultaneously for the temperature effect, This new bumability factor@)) is expressed by: Ca0f.B. (T°K)4. A higher value signifies a poor mix. It is claimed that this approach would provide a rapid and reliable evaluation of cement-raw-mix. However, for practical purposes, the characterization of the raw-mix may be evaluated at three significant temperature intervals: SOO-lOOO”C, lOOO”-1300°C and 1300-1450°C. Under standard conditions of temperature and time gradient, these could supply a more useful resuM4).

4. REACTIVITY 4.1 Definition The reactivity of raw-mix is defined by the overall rate of chemical reactions among the represented constituents of the raw-mix, attained on burning it at a certain temperature for a certain time, i.e. I& = f (T,@; this parameter, however, has no effect on reactivity when melt is formed above 1300°C.

4.2 Factors Affecting Reactivity The reactivity of raw-mix is influenced by: 1. Physiwchemico-miner~o~~~~~o~c composition as explained in Section 2.3. 2 . Chemical process of clinker mineral formations as explained in Section 5.2 and 5.3.

28 Progress in Cement and Concrete

4.3

Division and Characteristics

Reactivity may be divided into two groups: (a) Low reactive raw-mix (b) High reactive raw-mix Low and high reactive raw-mixes show marked influences on process and clinker character. This has been shown in Table 5. The above characteristics chart reveals that a high reactive raw-mix is always preferred.

Table 5. Characteristics of Low and High Reactive Raw Mixes Characteristic

Low reactive

High Feed consumption Sintering temperature figh Burning period Long Molecular dispersion rate Low Diffusion process rate Low Clinker mineral formation rate Low Unsatisfactory Mineral formation Ring/coating formation Uncontrollable Kiln Capacity’ Larger High Fuel consumption

High reactive Normal Comparatively low Relatively short High High High Desirable Adjustable Optimum Economical

4.4 Determination The reactivity of raw-mix may be determined@@ from the chemical composition and sieve analysis of the raw material and sieve analysis of insoluble residue. An empirical equation proposed to determine raw meal reactivity may be written as: J&c 1 (19) R, = S r2 r2 C “5 3 ma% = 9 where L

= liquid-phase quantity in the clinker = equilibrium C3S content in the clinker = average particle radius out of 25% of the coarsest ra&meal 5 particles = average particle radius (only retained on 40~ sieve) of the % insoluble residue of the meal in HCl s = computed for reference raw meal The proposed method neglects the errors accounted for minor constituents, liquid-phase quantity, alite contents and insoluble residue. These simple aperiments, therefore, are suitable to investigate the reactivity of raw meals collected from the same localities. C3L

Burnability

and

Clinkerization

29

6. REACTION SEQUENCE The course of reaction inside a rotary kiln has been of great interest to the cement technologists, since the kiln is computer-controlled and, obviously a mathematical model to explain the reaction process is to be constructed in order to find a logical relation between the process variables.

5.1 Kiln Temperature Profile It might be worthwhile to have an idea of the specified zones which give a clear picture of the reaction sequence in burning a cement raw-mix at critical zones to finally obtain the clinker through a complex physico-chemical transformation. The zones are conceptually defined by the temperature ranges and reaction profiles shown in Table 6. Table 6. Zones Defmed

Zone

by Temperature Ranges and Reaction Profiles

Temperature range “ C

Reaction profile

I

up to 200

II

200-800

Evaporation (slurry drying) Preheating (dehydration,

No-1100 1100-1300 1300-1450-1300 1300-loo0

dehydroxylation and fiit appearance of new phases) Decarbonization (calcination) Exothermic reactions sintcring COOlhg

III

IV V VI

The material and gas temperatures as well as reaction zones are differentiated and illustrated in Fig. 1.

5.2 Basic Reaction-Experimental Observations There is a glaring lack of clarity in the understanding of the reaction sequence of cement raw-mix in a kiln, owing to the wide variations in raw-mix physicochemical compositions, kiln operating conditions and, the practical difficulty of taking out hot samples from different points of kiln for study in the hot condition. One of the very early studies(‘Q demonstrated that in the burning process, dehydration, dissociation and decarbonation of the raw-mix components proceeds simultaneously with the formation of new phases. Subsequent experimental observations by different investigator@~ 619 7i-74) have been summarized in Table 7(79. These studies also revealed the following phenomena: 1. The first aluminate phase “CA” is fomed at lower temperatures (5%-600”C) and this, in turn, combines with free CaO resulting in the

30 Progress in Cemeai.anU

Concrete

Dry proccr

0

1 and11 I -2000~

Zones II

kiln

111 111

IV IV

v v

Wet proara kiln tsoo-

% otthe kiln length from. feed end

Figure 1. Material and gas temperature as well as reaction zones in wet and dry process rotary kiln. formation of an intermediate phase &A7 and finally it converts into Cpl above !WO’@). 2. The formation of C&S as an interm&ate phase is likely but dependent on the nature of raw materials used@s* 72+ ‘3). 3. The ultimate formation of C&F at higher temperatures (1300-14WC) is consecutively followed by the appearance of ferrite phases (CF and C2F’) at lower temperatures (800-900”C)(72~., Parallel and/or subsequent plant studieso679) on the reaction sequence in the clinkerization process led to the following observations, which also confirmed the above findings: 1. The reaction sequence of raw-mix is almost identical in dry, semi-dry and wet kiln.

Table 7. Clinkerization Reaction Sequence as Observed in Some Investigations Temperature range (’ C)

Reactions Ref. 71 Commencement of compound formation 550-600 and reaction products CA

Ref. 72’

Ref. 55

800

up to 800 CA + CF

C2F

(CF appears at 800-900) Commencement of decarbonization Coexistence of free lime with other phases Formation of C;S Formatron of t&A, Appearance of C,A Appearance of ajumino-ferrite Appearance of liquid phase Formation of CsS

700-750 950 950 950

-

9cKLllOO 1000-1100 1100-1200 1300-1400 low-1100

550 C2S

+ different aluminates + C2

600 -

Ref. 74

800-900

!900-1000 1100-1200 1100-1200 1200 1200-1450

(A.F) 900 550 550

-

1280 1280

Ref. 61

600 CA+ CL2A, + C$

600 17% at lCKtO+ 600 600 1000 At higher temperature

Free CaO does uot occur in cxccss of 2% until the entire A1203, Fe303 amI SiO3 have combined with CaO. + CoUktbphases at different temperature8 as reported: GF (8W’C). C2F + CF (900°c), C2S + C2S + C2AS + CF + C2F(l@)00C), C3S + C3AS + CsAdC12A7) + C3S + CF + C3F (1 loO’C), C3S + C3A + C3A3 (Cl3A7) + C3F + C3S (lZOO”C), C3A + C3S + C3S + C3F (13OO’C)

l

and C,A+CsS+C#+C&F (MOO*C)

3 2 Progress in Cement ond Concrete The dissociation and decarbonation of raw-mix components start at 550600°C. The CaO formed during decarbonation reacts with other components simultaneously in such a way that about 2% CaOr at 800°C and about 17% at complete decarbonation temperature (1000°C) remain unreacted(‘Q . 3 . The first detectable phases CA+C12A7+ C$S were noticed at 700°C. The amount of these phases increases with temperatures up to 900-lOOO”C, when poorly detectable C&S and some C4AF/C2F are traced(‘6). 4. In some other investigationsc 7rv7*), the first phases detected are CF + CA + CS which are subsequently converted into clinker phases with rise of temperature in accordance with the following scheme:

2.

CA C12A7 C3A CF C4AF C2F CS c3s2 c2s 5 . a-Fe, Fe0 and Fe203, along with the formation of a-wollastonite almost concurrently with P-C2S are detected from a series of charge samples@@. 6. An extensive study was rnade(‘9) after comparing with five kiln charges/coating and a reaction sequence was derived accordingly (as shown in Fig. 2), which further confirmed the above observations. Row mater ids

Intermediate phase

Clinker phases

Figure 2. Reaction sequence in cement rotary kiln.

Burnability and Ciinkerization 7.

33

The solid-state reactions are almost complete at a temperature of about 1300°C and a melt phase appears.’ The melt phase contains a complete melting of CsA + C&F, and partial melting of CzS and CaO with the incorporation of constituents such as MgO. The formation of CsS is activated through the diffusion of CzS and CaO in the presence of melt. The final clinker phases appear with the formation CsA, C&F, C#, CsS, MgO and glass after crystallization of the residual liquid.

5.3 Secondary Reactions-Presence of Minor Constituents and Catalytic Additives Apart from the controversial appearance of intermediate phases like CS, C& CzAS, various investigators identified different intermediate compounds or complexes stably co-existing in the presence of various catalytic additives or minor constituents in raw-mixes, at temperatures ranging from 100 to 1300°C. Important findings of various investigators after the compilation of the observations, are summarized in Table 8c”). In addition to the above findings, the formation of spurrite 2C#. CaCOs is detected from the coating samples when they were withdrawn at temperatures ranging from 680-lOOO”C, particularly in the presence of halides and alkalies in the raw-mix. The mechanism of its formation has been studied by different investigators and explained on the following lines: 1. 2. 3.

4.

5.

Low melting alkali/calcium carbonates might form and promote spurrite formatiotW. Spurrite formation is perhaps due to faulty control of burning conditions(s2). The most logical argument(79) probably was that spurrite formation was initiated by CzS since spurrite + quartz or spurrite+&CzS were not found. Spurrite, after reaching the very hot atmosphere of the kiln where CO2 content is low enough, decomposes into CsS or CzS + CaO. Thus it helps in CsS formation. The formation of C12A7. CaCOs has also been reported along with spurrite(

Gehlenite, C2AS (or melilite with very little magnesia), was detected in the coating sampledsO) from a kiln and it has been suggested that CzAS is the product of the reaction between kiln refractories and clinker-forming materials. C2AS is particularly found where high alumina bricks would normally be used, but akermanite and melilite would also be expected when dolomite or magnesite bricks are used. It is reported that magnesio-ferrite, MgO/Fe0.Fe20s, is formed when raw-mix is burnt in a coal-fired kiln under reducing condition@). From the above studies onthe reaction sequence, it is evident that the clinkerization process involves the ultimate stabilization of CsS, &C&I, CsA and C&F through various stages of decomposition, interactions and crystallization. The stability ranges of the more commonly encountered phases

34 Progress in Cement and Concrete Tab& 8. Intermediate Compounds Catalytic additives or minor constituents

Intermediate compounds

Stability range Remarks

Phosphorous pentoxide

7Ca0.P20,.2Si02 9 Ca0.P,05.3Si01 5Ca0.2P,05.Si02 5 .3Ca0.2Pz05.Si02 7.3 CaO.P,Os.SiOs 27CaO.P,0,.12SiOs Ca3PW3F

up to 1450°C

At 1450-1500°C they dissolve, recrystallizing C2S + C3S. Some compounds may remain in clinker without decomposition. The presence of CsP and C,P is possible, if P,Or >0.5

Chromium oxide

cao .cr203

1ooo-1400°c

Cr in clinker is normally present as Cr3+, Cr5+ and Cfi+ and rarely as Cr2+

CaOCrO, ZCaO.Cr,Os 3Ca0.Cr205 9Ca0.4Cr0,.Cr203 3Ca0.Cr203.3Si02 18Ca0.10CrO~.Cr,03 7Ca0.Cr,0s.2Si02 4Ca0.3Al,0s.CrOs

Sulphur compounds

3(CA).CaSO, Z(C,S).CaSO, 3(CsS).CaSO, CaSO,. 1.75SiOs 2CaSO,.K,SO, 3Na$3O,.CaSO,

900-1400°C

Alkali compounds

Na2WC03)2 Na2C0s.2Na2S04 CaS0,.3Na2S0, 2CaS0,.K2S04 K2CWW2

up to 780-830°C

4(Qdr0.6)

Strontmm oxtde 0.3A120,.SOj

The resultant eutectic melt helps the development of Na20.Si02, Na20.Ca0.Si02, 8Ca0.2Naz0.5Si02, 4 CaO. 2 Na,O, 3Si02, Na,O. A1203, K20.A120s, which decompose of melt at 1 lOO-1200°C The stable phases at 1300-1450°C are KC23SL2, NGA, KCBA3, Na#O.,, KsS04 and 3K2S0.,Na2S04

.

Burnability and Clinkerization 35 (Table 8 Contd.) Catalytic additives or minor constituents

Intermediate compounds

Magnesium oxide

Ca0.Mg0.Si02 2CaO.MgO.Si0, 3CaO.Mg0.2SiOz 2Ca0.Mg0.2Si02 7Ca0.Mg0.2Al,0j 3Ca0.Mg0.2Al,0J MgO.Alz@ MgO.Fe,O, Formation of

Stability range Remarks

1200- 1350°C On raising the temperature beyond 1400°C the magnesium compounds decompose and MgO crystallizes from the melt as periclase MgO can include as isomorphous impurity FeO, MnO, and ZnO upto 10% each and about 0.1 Vo (atomic Ti, Ni, SC, Hf.

(CZ%.6.Gwh.4

between CzS and CjMSI Sodium fluoride

2CaO.SiOz.NaF

1000°C

Calcium fluoride

2(C2S).CaF2 2Ca0.Si02.CaFz 3 (CS).Caf, 1 ICa0.7Al,O,.CaF, 4Ca0.4SiO,CaF,

950- 1 170°C

3Ca0.3A120,.CaF, Calcium chloride

2Ca0.Si0,.CaC12

IllO- 1185°C

Decomposes at I 185°C into C,S + C2S + Liquid. This phase in clinker may have solid solubility of 4-5% Al,O, and 364.5% NazO. It has also been considered as a solid solution of CaF, and C$ with a formula 3(CrS)CaF2

1505°C 1084-I 100°C

C$.CaC& - Q -C2S + CaCl,. Cl- may enter into the structure of aluminates and aluminoferrites. The formation of chlorosilicates of different basicity is reported in the system CaO - SiOz - CaClz

36 Progress

in Cement and Concrete

are given in Fig. 3t61) and approximate reaction sequence in clinkerization is given in Fig. 4.

CaCO3

Cristobati te Qurrtx C AS2 C2AS Ct(AFl CA

C12 At C3A c s C2S C3S Co0 Boo

900

loo0

mo

1200

1300

1400

(500

.C

Figure 3. Stability ranges of different phases during clinkering.

600%

279 Clchydmtion and dehydroxy lation

- 50%

1ooo”c

Dccarbonation 660°C Break d o w n o f aluminosilicates

1

950°C A1203~Si02fF~03

sso”c Solid-state

reactions

1280°C

1

1,

f c2S+CA+C12q+hrritetCaOt 12eot 14w”c

Liquid

phase

I ss +t+S melt

sintering 1000’C

Cooling

l3OO’C

I S+C2S+C3A

+C#

Figure 4. Approximate reaction sequence in clinkerization.

BumczbiMy and Clinkerization 5.4

37

Clinker Phases-Effect of Cooling Rate

It has been seen that the clinker phases, as well as the melt formed during sintering, get affected by the rate of cooling. It is reported that the best clinker is obtained by cooling slowly to 1250°C followed by rapid cooling. The effects of the cooling rate on the clinker phase and their properties are summarized in Fig. Vu) and the demand for rapid cooling (18-20”Umin) has been suggested.

Figure 5. Effect of cooling rate on cement properties and phases 6. THERMODYNAMIC CONSIDERATIONS The application of thermodynamics to a clinker-forming system is a highly complicated one, since the system covers an extremely complex physicochemical transformation in the presence of solid-liquid-gaseous environments. The presence of raw-mix-gaseous products, clinker phases, and clinker liquid in the kiln atmosphere at regular temperature intervals, makes the system so complicated that a clear understanding of thermodynamics is really impossible. However, a systematic investigation(s) with simple oxide and carbonate systems excluding minor constituents such as RzO, MgO, PzOj, SO3 has so far been made from temperatures below 700°C to at least 1200°C. The most probable basic reactions of clinker formation at temperatures around 700°C area:

38 Progress in Cement and Concrete CaCOs + Alz0~.2Si0zL2Hz0 = CaO + AlzOs.2SiOz + CO2 + Hz0 (20) SC&O3 + A120s.2Si02 = 2@ - 2CaO.SiOz) + CaO.AlzOs + SC02 (21) 5CaO + A120s.2Si02 = 2(/3 - 2CaO.SiOz) + CaO.Alz03 (22) 40 CaCOJ + 7(AlzO+2Si@) = 12Ca0;7AlzOs + 14 (13 - 2CaO.SiOz) + 40

(23) F-%aO.SiO + CaO = 3CaOSiOz 7CaCOj + A1203.2Si02 = 2(3CaO.SiO2)

WI + CaO.AlzOs + 7CO2

(25)

The following conclusions can be drawn from the study of the above reactions: 1.

The transformation of kaolinite to metakaolinite [(es. 20)] is thermodynamically favourable during the early stages of the reaction. 2. The formation of CsS is thermodynamicahy possible when the reaction between &CzS and lime [(eq.(24)] or calcite and metakaolinite [eq.(25)] is subjected, while the interaction between C2S and calcite does not thermodynamically yield C$S. 3. The reaction between calcite and metakaolinite is more favourable to yield CtzA7 and CzS [eq.(23)] than CsS and CA (eq. [25)]. 4. The formation of /3-CzS is thermodynamically possible when calcite or lime reacts with metakaolinite [eqs.(21), (22), (23)], while the interaction between calcite and kaolinite does not thermodynamically favour /3-C2S formation. 5. The formation of CsA from CA+ CaO or CA + CaCOs is thermodynamically impossible, while CA is formed by the interaction between calcite or lime with metakaolinite [eqs. (22) and (25)]. It is suggested that the formation of CsA requires a melt phase. 6. The formation of C&F from ferric oxide and calcite or lime is possible over the entire temperature range but the conversion of C2F into C&F in the presence of CA and calcite or lime is impossible, which apparently suggests the presence of the melt phase. It should be pointed out here that these reactions have little practical significance, since their existence in a kiln feed below 700°C is limited. Hence, the study of mechanisms and kinetics of clinker phase-forming reactions is the great significance in order to establish a relation which could predict an improvement in the burnability of raw-mixes and a reduction in the energy requirement of the clinkerization process.

7. REACTION MECHANISM AND KINETICS

7.1 Simply Binary System The simply binary systems were studied by different investigator+92). The system consists of calcite/lime with silica/quartz or alumina under isothermal conditions. The kinetics of simple system is controlled by solid-state diffusion. The following common observations can be made from the study of the simple system mechanism:

Burnability and Clinker&ion 39 1. The solid-state reactions between lime and silica or lime and alumina are diffusioncontrolled. These reactions are distinguished from reactions of other categories in which the rate is controlled by the movement of reaction interface, by nucleation and crystal growth or by an empirical order of reactions(B). In the diffusion process, Ca2+ ions are the diffusing species. Counter-diffusion of other ions like Si4+ or AP+ has not been firmly established(ms92). 2. Originally, it was regarded that the diffusioncontrolled reaction in solid phase followed Jander’s equation(s5) (l-S&& = (k/rl)t (261 where 01 is the fraction reacted in time “t”, k the rate constant, and r the radius of the particles. Recent studies(~~~) on the formation kinetics of clinker phases from the constituent oxides indicate that these reactions are better explained by the diffusion-controlled equation established by Ginstling and Brounschetein: F(a) = (1 - 2/3a) - (1 - (r)1/3 = (k/rs)t (27) where a, t, k and r are the same as in eq. (26). 3. In another study, the reaction mechanism was explained by a suitable model. The model is composed of intermediate phases, the outermost bearing Ca-rich (silicates or aluminates) and the innermost bearing Si/Alrich. However, C# is formed directly without intermediates in the system of CaO - Si02. Even CsS in the same system is formed by a reaction between C2S and Cao(9**!Q and CsA in the system CaO - Al203 is formed through intermediate phases CA2 and t&A7 with other reaction products like C& and CA. 4. The rate of C$ formation is significantly affected by the chemical nature of the starting ingredients, the firing temperature, the rate of heating, etc.t91).

7.2 Complex Systems The complex systems consisting of limestone and clay minerals (kaolinite and illite)(73~~~95), lime and alumino-silicate like siliceous clay, aluminous clay or granulated slag@@ or lime-alumina-iron(~), are more or less comparable with the cement raw-meal composition. Thus, the kinetics study of complex systems is more significant in explaining the exact condition. These studies confirmed all the findings reached with the simple oxide system but in addition, the following observations were made: 1. The first appearance temperature of CrS is dependent on the nature of the raw materials used. The effect of raw materials on the appearance temperature is indicated in Table 9. It is clear from the table that the variation in raw materials used appreciably changes the reaction mechanism and appearance temperature of CJS formation.

40 Progress in Cement and Concrete Table 9. Effmt

of Raw Mater& on the Appearance Temperature of C$

System

Appearance of C$(“C)

Lime - clay Lime - silica Marble - Kaolinite Marble - Illite

1300 1100 1100 1000

2.

During the experimental work(%) was modified as:

temperature

Reference 73 85 9s 95

on complex systems, Jander’s

equation

(1 - 3&)N = (k/rz)t (28) where N = tan 9 and $ is the angle between the straight line and ordinate, varies widely with the raw materials, temperature and time of burning. The following conclusions were drawn from the values of N and K: (a) Contact reactions, followed by diffusion process, are predominant at successive periods of heat treatment. (b) Jander’s type equation is not effectively applicable to the burning of portland cement clinker as “k” takes minimum values at medium temperatures and the values of “N” become very high at high temperatures. (c) A more realistic kinetics picture would be obtained if the reaction ratios and the formations of CzS (2C + S+C$j) and CsS (C + C+C3S) are separately examined below and above 125O”C, respectively. 3 . In one of the recent additions@*) to the study of solid-state kinetics of clinkering reactions, it was observed that three stages should be conceived in the solid-state range of reactions: (a) Dissociation of limestone and clays; (b) Formation of calcium silicates and alumino-silicates of lower basicity (pseudo-wollastonite, gehlenite, anorthite, etc.) which follow the mechanism of volume diffusion of Ca2+ through reaction zones; and (d) Formation of &C2S and aluminates including CsA following the mechanism of surface diffusion. The above reactions in the range 1000-l 100°C followed the pseudotopokinetic equation, given as: K‘=

LIn al-m

l-cr l+oL

(29)

where m is the heterogeneity factor equal to 0.98, signifying that solid-state reactions occur in the kinetic field. Further, in case of limestone slag mixes@), two steps, i.e. dissociation and clinker mineral formation, were followed, since the intermediate phases (calcium silicates and alumino-silicates) are already present in the slags.

Burnability and Clinkerization

41

7.3 Activation Energy in Solid-State Reactions The activation energy in solid-state reactions can be calculted from Arrhenius’ equation: .K = A.exp (- Ea/RT) (30) where Ea (kcal/mole) represents the activation energy, T(“K) is the temperature at which the process occurs, A is a (quasi) constant< called frequency factor, equal to rate constant, K when T = 00. 7.3.1

Activation Energy of Limestone Dissociation

It is well established that the activation energy of limestone has a strong effect on the activation energy of clinker mineral formation. So it may be worthwhile to have an idea of the variations in the activation energy of limestone dissociation in the process of clinker mineral formation. The commonly accepted value for the activation energy of CaCOs dissociation is 45 kcal/mole(*), while 44.34 and 42.32 kcal/mole were reported for the decomposition of calcite(tsQ and analytical grade CaCOs@@, respectively. It has been observed(ts@ from the study of plant raw-mixes that the decomposition of limestone occurs in two stages and ultimate activation energy in all the cases is higher than that of pure calcite (Table lo(rOr)). The activation energy of limestone dissociation varies from 30 to 60 k&/mole. The variation is due to the associated impurities and proportion of calcite in raw-mixes (Table 10); for example, the presence of impurities like aragonite leads to an increase in the energy barrier for decomposition, while clay minerals reduce the activation energy. Table 10. Activation Energy of Calcium Carbonate Decomposition system Plant raw-mixes A

B C D Calcite + Kaolinite 3.5:1 6:l

9:l Calcite+C,AS (1.5:1)

Ea (kcal/mole) Lower stage

Upper stage

-

53.72

29.82 25.61 34.97

47.41 45.79 49.19

Ref. 100

34.36 35.78

98

41.61

98

30.35

Increasing the proportion of calcite in raw-mix increases the activation energy, resulting in decreasing the rate of &qS formation and lowering the lime combination at 1300-14OO”c<9e,.

42 Progress in Cement and Concrete Using different proportions of CaC03 in the calcite-kaolinite mixes, for example, mix ratios of 3.5:1, 6:l and 9:1, it was revealed(*) that 1. The rate of dissociation of carbonate and clay was two orders higher than that of the new mineral formation, which confirmed that the rate of dissociation does not limit the mineral formation process. 2. J3y increasing kaolinite in mixes, the activation energy for the dissociation of carbonates decreases due to the presence of water vapour from clay decomposition. 3. By increasing the carbonate content in mixes, there is an increase in the energy requirement for dissociation, as a result of which the possibility of volume diffusion of Ca2+ decreases. Consequently, the reaction products in the three mixes are different: 3.51 mix - CaOr, metakaolinite, pseudo-wollastonite and gehlenite, 6:l mix -@-C$ and CsA in addition to the above phases. 9:l mix-CaOr, /3-CsS and CsA. 7.3.2 Activation Energy of Cllnku Mineral Formation The reported values of activation energy for the formation of the major clinker phases are presented in Table 1 l(lol). The following conclusions may be drawn from there: Table II. Activation Energy of Major Clinker Phases Phases

Ea (kcal/mole) Reaction system

-200

c3s

8-W

C3A

C.,AF

127.76 14.63 61.57 42.85 25.10 36.66 39.40 48.41 119.34 86.60 35.50 99.62

CaC4-alumino silicate like clay and slags CaO - SiOl (3: 1) CaCO:, - Kaolinite CaC03 - Kaolinite (6: 1) CaCO, - Kaolinite (9:l) CaC03-CzAS(1.5:1) CaCO, - QAS(4: 1) CaO -XiOl (2: 1) CaO-A1203 (3:l) CaC03 - Kaolinite (6: 1) CaCO, - Kaolinite (9: 1) CaC03-C2AS (1.5:1) CaCO, - CIAS (4: 1)

104.68 70.98

CaO - A&O3 - Fez03 C3A - CF

References 96 92 95 98 92 90 98 97 97

1. Activation energy of each of the major phases varies significantly,

depending on the ingredients in the raw-mix as well as on the proportion of the ingredients of the same raw-mix.

Burnability and Clinkerization 43 2.

Activation energy of C3S formation in solid-state reactions is much higher than the activation energies reported for serf-diffusion of Ca in CaO (33.946405 kcal/mole)(i0zJcs), whereas the activation energy of CzS formation, by and large, agrees with the self-diffusion of Ca and CaO. 3 . Activation energy of C3A formation comes close to the activation energy of self-diffusion of Ca in CaO only in certain cases (like CaOAl203 and 1.5:1 mix of CaCOs-CzAS). Thus, the reaction appears to occur via Ca diffusion into AlzO3. 4 . Activation energy of C&F formation is similar to that of the diffusion of Fe into Fe203(g), while in the reaction between CsA and CF, the ratecontrolling step may be the diffusion of Ca into CF to form Ca-rich ferrites or the diffusion of Ca or Fe (or both) into the aluminate phase. 5 . A comparison of activation energies of carbonate dissociation and /3C2S and C3A formation in calcite-kaolinite mixes (6: 1 and 9: 1 in Tables 10 and 11) clearly shows the inverse relationship between the energy barriers of dissociation and mineral formation in the same systems.

7.4 Diffusivity in Solid-State Reactions It has been conceived that the solid-state diffusion process is controlled by the diffusion of Caz+ ions in the acidic/acceptor oxides. The diffusion phenomenon proceeds in three wayW% (a) Surface diffusion (0,): It is followed by the mass transfer on the external surface of the grains of acceptor oxides. (b) Joint diffusion (DJ: It is governed by the mass transfer along the surface joining the grains of the acceptor oxides. (c) Lattice or volume diffusion( D, ): It is controlled by the mass transfer inside the grains of the acceptor oxides. Since a higher level of free energy is present inside the lattice than at the surface or joints, activation energies of the process at the surface (Es), at the joints (Ej) and inside the lattice (Ea), increase accordingly under isothermal conditions(ies). Therefore, Ea > Ej > Es (31) It is then evident that the lattice or volume diffusion being the slowest one controls the overall stationary process under isothermal conditions. The diffusion coefficient (DJ can be expressed by a similar equationQO), where the rate constant K is equal to D, and the frequency factor A is denoted by D,: D, = D, exp ( - Ea/RT) (32) Some data on the diffusion coefficients of the clinker phase formation reported earlier@QQ are presented in Table 12(iOi) in a selected summarized form.

PP 3 3

Table 12. Diffusivity in Clinker Phase Formation Reactions Diffusivity (m2 h-’ System 45CaCOI

1000 “C

1100 “C

-

-

-

5

10

CZS

-

0.71 0.68 0.77 -

1.02 0.79 1.02 0.28

-

2.18 0.94 1.60

-

20

58

110

Layers + kaolinite

CaO + kaolinite

c3sz

cs C3A

45CaC03 + illite CaO + illite

-

-

c3s c3s,

CAS;

1200 “C

-

1250 “C

x

-

1300 “C 17

-

s. R

109) 1350

1400 “C

1450 “C

Ref.

20

90

200

94

-

-

-

95

180

340

2000

94 95

-

-

-

a11 0.10

-

-

-

-

-

-

-

0.03

-

-

-

-

-

-

-

9 P ;; & 9 2 2 ip”

Burnability and Clinkerization 45 The following conclusions can be drawn from the data on diffusion coefficients (Table 12). 1. Values of diffusion coefficients are substantially different even in identical systems under isothermal conditions. 2 . Diffusivety increases considerably with a rise in temperature. 3 . Diffusivety is strongly dependent on the nature of diffusion and hot species.

7.5 Liquid Phase Sintering in Clinkerization In all the investigations reported earlier, the presence of melt phase was neither mentioned nor taken into account in explaining the reaction mechanisms, whereas experimental studies and phase equilibria considerations of even the simplified clinkering systems indicate the presence of a liquid phase at clinkering temperatures. However, the possibility of a liquid phase modifying the reaction mechanism of the formation of C3S was critically viewed(%) and the observation of an anamalous activation energy of ($5 (see Table 11) was also supported by this presumption. As a result, later studies on the kinetics of clinker-phase formation reaction were more concerned with the ultimate formation kinetics of C3S in the presence of a melt by a reaction (eq. 33) than the preceding stages of reaction which were, by and large, in the solid state. c+c$3

~ melt

cjs

(33)

The investigations carried out on C$S formation were based on experiments with briquettes of varying clinker-phase compositions juxtaposed and fired with or without a glass-layer inserted in between or adjacent to the briquettes. Whatever, the composition of the glass, it corresponded to the 1338°C eutectic of the CaO - A1203-Fe20J-Si02 systems!or a molten coal ash or a slag of appropriate composition. Studies conducted so far on CsS formation in the presence of a melt phase can be broadly classified and fired with our without a glass-layer inserted in between or adjacent to the briquettes. Whatever the composition of the glass, corresponded to the 1338°C eutectic ofthe CaOA1203-Fe203-Si02 systems, or a molten coal ash or a slag of appropriate composition. Studies conducted so far on CsS formation in the presence of a melt phase can be broadly classified under three group: (i) Russian (ii) Japanese (iii) Danish 7.5.1 Russian Group Investigations The important findings by the Russian group of investigators are summarized belotite@: 1 . The clinkerization reaction in the presence of a melt phase has been conceived to include three independent physico-chemical processes(t@% (a) Dissolution of the minerals of raw materials and the product of solidstate reaction in the liquid phase.

46 Progress in Cement and Concrete

2.

3.

(b) Diffusion of Caz+ and SiOc4 ions into the melt. (c) Crystallization of new phases. Out of the above three steps, the first two relate to the burning process and the third to cooling. The rate-controlling phenomena in burning and cooling are different. The rate-controlling phenomenon in burning is the dissolution of clinker minerals in the liquid phase, which does not proceed through the migration of the elementary particles (atoms, ions, molecules) into melt, but proceeds by means of separation of indivdual blocks of up to 1 pm size from dissolving crystals and polycrystaWOs) and is represented by the equation v,=vK’l’ ND exp [ - (E + AE)/RT] ’ a

(34)

where Vp = rate of dissolution of CaO or C$S in the liquid phase of clinker. Vx = rate of clinker formation = dissolution time s’ = diameter of the elementary particles = frequency of elastic vibrations of the elementary particles in the YO solid state, N = multiplication factor considering that lo9 - 1010 particles separated as a whole block fit for one molecularly separated particle, E = activation energy of dissolution R = gas constant D = certain effective size of the grain AE = activation energy of the block seperated from the grain T = temperature 4. Crystallization of clinker liquid phase in the range 820-1450°C proceeds very quickly during clinker cooling. The rate of crystal growth is the ratedetermining phenomenon, when only one phase crystallizes, and the rate of crystal nuclei formation becomes the determining phenomenon when more than one phase crystallizes out. 5 . The activation energy for the dissolution of CaO and CzS in the liquid phase was reported to be 150 kcal/mole which is close to the activation energy (200 kcal/mole) of the C$S formation from the solid-state diffusion(%). 7.5.2 Japanese Group Investigations Unlike the Russian investigations, the Japanese stud$t@) was basically concerned with C,S formation only. In order to investigate the mechanism of reactions on the boundaries of CaO-melt and CzS-melt, a powdered glassy phase was sandwiched between two compacted discs of CaO and /3C$S. After subsequent heating and air quenching the specimens, the following conclusions were reached:

Burnability and Clinkerization 47 The rate of CJS formation was dependent on the quantity and viscosity of the melt phase, fineness of the particles of reactants and temperature, the last parameter, in turn, helping in increasing the amount of melt, decreasing its viscosity and increasing the diffusion coefficient. 2 . The broad mechanism of the reaction includes the dissolution of CaO and ClS in the melt phase, supersaturation of the liquid with counterdiffusion and crystallization of the CJS phase. 3. The apparent activation energy of dissolution of the solid phases depended on the quantity of the melt phase, as indicated in Table 13. The Ea-values obtained with a larger amount of the melt phase were closer to the values reported by the Russian group, while the values with a lower amount of melt were closer to those expected from the viscosity data. These results led to the conClusion that counter-diffusion in the melt is directly related to activation energy and unexpected greater values are due to a change of tortuosity of the path for diffusion. 1.

Table 13. Apparent Activation Energy Melt phase (in wt% of CaO and C2S)

Particle size r Olm)

Activation energy E, (kcal/mole)

30 30 15 15

5 7.5 5 7.5

123 121 42 42

4. The rate of reaction could be represented by an equation of Jander’s type with a clear physical meaning where dissolution was considered in spite of inward diffusion being assumed by Jander. (l-36)2 = y K.t = K.t = F (ar) where a = reaction ratio D = diffusion coefficient AC = concentration gradient across the diffusion layer T = time = radius of the dissolving spherical particles r0 K = reaction rate constant 7.5.3 Danish Group Investigations Danish investigations were more or less an extension of the studies of the 3apanese group with a parallel attempt to correlate the kinetic data with phase equilibria considerations of simplified ternary (C-A-S) and quaternary (CA-S-F) clinkering systems. The first reported investigations in this serie#*O) experimentally confirmed the mechanism of diffusion in the melt as the ratecontrolling step in the interaction between portland cement clinker and molten

48 Progress in Cement and Concrete and molten coal ash. While in this study there was an extension of the liquid phase in the clinker by a liquid of different composition (ash), the subsequent studyoll) was restricted purely to the ternary system (CA-S). The above studies revealed that: 1 . In this clinker and ash reaction, the diffusion coefficient of CaO in the melt at 1500°C was about 5 x 1O-6 cm2/s and the activation energy obtained from the diffusivety measurements of different clinker-ash combinations was of the order of 42 f 17 k&/mole, which was close to the value reported by the Japanese group. 2 . In the pure ternary system, however, D was observed to be 4.5 x lo-’ cm%, when the tortuosity of the diffusion path was taken into account and the diffusion coefficient was obtained as 7.5 x 1W7cm2/s, although the latter value was considered to be less accurate. Irrespective of the difference between the values, the estimated D in the ternary system was found to be significantly lower than in the quatemary system mentioned above. 3 . Diffusivity increases with an increase in temperature and changes with composition. When the free CaO is more, diffusivity is higher and free CaO is essential for the ultimate formation of CsS. As a further development of these studies, a simple model was established for diffusioncontrolled reaction between spherical CaO particles and portland cement clinker at 15OO”C(*tz), experimental results from which agreed well with those predicted from data obtained in the independent diffusion experiments. According to this model, if the fraction of CaO reacted is represented by x, the time dependence of x is given by: (1 + flx)z” - 2/3/3x = 1 - [@or B D A C)/di)] (t/ri)

(36)

where /3 = a function of composition and porosity a! = volume fraction of liquid D = effective binary diffusion Coefficient of CaO At = concentration gradient of CaO across the reaction layer 4 = density of CaO t = time r. = radius of CaO particles Applying the above model, it was observed that at 15OO”C, for a burning time of 20 min, the maximum size of CaO particles that can completely react is !+m corresponding to calcite grains of 120 pm size. Recently, a sandwich technique was developed(ii31 i14) for determining the rate of clinkerization in systems like C-A-S-F, C-A-S-F-M and C-A-S-F-MCaF2 with an advantage over powder technique by eliminating particle size effects. Moreover, kinetics are simpler and often easier to interpret. The rate constants were determined from the equations: X2 = Kt

K -- 2DmCx- An . H z

(37) (38)

Burnability and Clinkerization 49 H = l/(C,C,)+l/(C,-c,) where X K t Dm

(3%

= = = =

thickness of layer of CsS + C + melt and CsS + C# + melt) rate constant time effective binary diffusion coefficient for CaO with counter-diffusion of SiOz + AlzOs, etc., in the melt cx = weight fraction of the melt in the CsS layer An = difference in the CaO concentration (weight fraction) for melt in equilibrium with CsS +C, and melt in equilibrium CJS + c2s weight fractions of CaO in the two briquettes Cl, c2 = = weight fractions of CaO in the CsS layer at the briquette-Icm Cb CsS interface and at the CsS-briquette-II interface = factor < 1, for the tortuosity of the melt 7 The rate of constants obtained in the different systems are given in Table 14(10’) which reveals the influence of temperature and composition on K.

Table 14. Rate Constants in Different Systems Composition (wtolo) II

System 1. C-A-F-S

CaO SiO, A1203

2. C-A-F-S-M

Fe203 CaO SiO, Al203

Fe203 MN 3. C-A-F-S-

System

M-CaF, System-2

I - 1350

73 21 4 2

67 27 4 2

73 21 3.3 1.7 1.0

67 27 3.3 1.7 1.0

2+0.5% CaF, 2+ 1.0% CaF2

Rate constant K x 108 cm% at temperature (“C) 1375

1450

1500

-

12.2

22.4

31.5

7.4

9.1

18.0

26.6

17.4

-

21.3

38.6 42.4

It is clear from eq.(38) that the rate constant ‘K’ is composed of four factors. The factor ‘H’ is related to the microscopical distribution of the main components whereas the remaining three factors-a, Dm, and An-are rather insensitive to the distribution of main components but may be influenced by change in the content of other components or temperature, as discussed below:

50 Progress in Cement and Concrete 7.5.3. 7 Melt Content la)

The melt, depending upon composition, cannot exist in stable equilibrium with alite below a minimum temperature, Ta. The effect of composition on TE and o can be seen from Table 15. Table 15. Effect of Composition on ‘TE’ Composition

T,“C

and ‘a’

‘CY’ at 1400°C (T) (w/w~o)

C-S-A C-S-A-M C-S-A-F C-S-A-F-M C-S-A-F-M

> 1450 <1400 < 1350 = 1300

0 10-15 15-20 20-24

-Ti-Mn-N-K

1260-1280

24-28

Thus o, for a given composition, increases from zero to a finite value as T passes Ta. With the addition of M,F or impurities (Ti, Mn, K, N, etc.) in the main component, TE decreases and o increases at a certain temperature(T). 7.5.3.2 Diffusivety (D,,,!

The value for D, is = lO-‘j cmz/s at 1500°C. Dm increases approximately twofold for a lOO“C temperature increase. This corresponds to an activation energy about 45 kcal/mole. It has been seen that D,,, and alite formation rates increase by lowering the viscosity of the melt when a suitable modifier is added to the system. But this is not always true. Therefore, the effect of composition on Dm remains open to justification. 7.5.3.3 Driving force (An)

An is an index for the driving force for inter-diffusion of CaO and SiOl through the intertitial melt in the alite regions and for their growth. A positive definite value for An signifies that alite is relatively stable to lime and belite under the specified temperature and composition. The temperature dependence is connected with the fact that alite has lower stability temperature, To, typically at 12QO-125O”C, depending upon composition. At T c To, lime and belite are comparatively stable to alite and there can be no driving force or alite formation. Therefore, An = 0 for T 6 To The compositional influence upon An at a constant temperature T ( > To) can be deduced from the many relevant ternary and pseudoternary phase diagrams available. However, the value of An is so small that extremely accurate analysis would be required.

Burnability and Clinkerization

51

If two or more of the An influencing components are added simultaneously, their combined effect is expected to be simple superimposition of their individual effects.

7.6 Effect of Minor Catalytic Constituents on the Formation Rate of C&S The study(l14) on the rate constant in system C-A-F-S-M with CaFs (Table 14) can be reckoned to be the beginning in the clinkerization process. The addition of CaFz will, of course, change CL It may change the physical properties of the liquid and thereby D,,, and possibly An and H [eq. (38)J. It has been seen that an increase in CaF2 results in a corresponding increase in K (Table 14). It should be mentioned here that the effect of CaF2 on K, whether due to a non-linear relationship between An and CaFs or due to the influence of CaFz on Dm or due to a variation in 7, is not clear.

7.7 Phase Equilibria Considerations It has already been mentioned that in all Danish studies there was an attempt to correlate the kinetic findings with phase equilibria considerations of the simple ternary system C-A-S, the isothermal section of which at 1500°C is reproduced in Fig. 6 and the phases in equilibrium in different parts of the diagram indicated in Table 16(icr).

Figure 6. Isothermal section at 1500°C from CaO corner of system Cao-Al,O,,-SiO,

52 Progress in Cement and Concrete Tab& 16. Zones with Equilibrium in the 15tW°C Isothermal Section of C-A-S System Region 01 02

03 04

03

No. of Phases 2 3 2 3 2

Phases in equilibrium CaO (solid) + A - B (liquid) CaO (S) + C$ (S) + B (L) c2s (S) + c$3 (S) + c (L) C$3(S) + C&s) + C(L) C2S (S) + C-D (L)

From Fig. 6 and Table 16, the following conclusions have been drawn: 1.

With the inherent microinhomogeneity present in raw-mixes, the composition of microvolumes is likely to fall in different regions (0, -OS), equilibrium in which will be attained quickly, giving rise to phases determined by the equilibrium conditions of the local composition and a given temperature. All further reactions are restricted to the diffusion of the species between different regions for which reaction layers with recognizable boundaries will develop. 2. When a clinker passes through the burning zone of a cement kiln, the phases CaO, CzS are dispersed in a melt with concentration gradients in many geometrical directions. The reaction would thus result from diffusion between pockets of A (C + CsS) and B(C$ + CzS). The phase diagram shows a very narrow finger for the primary field of CsS, so that A and B compositions are similar (60.4% and 59.3% CaO, respectively, at 15OO“C). The size of the concentration gradients is very limited, which may be the reasons for sluggishness in the formation reaction of CsS or complete assimilation of CaO under practical conditions(tt2). 3 . As long as the melt composition in the belite cluster differs from C, CaO will diffuse into the cluster and correspondingly SiOz+ A1203 will counter-diffuse. In this manner, CJS will be dissolved at the boundary and CzS will precipitate. As a result, a cluster of CzS with increasing size is developed during the process(l15). 4 . The two-phase regions Or (CsS + melt) is developed when the threephase regions 4 (C+ C2S +melt) reacts with another three-phase regions 0, (CsS + CzS + melt). If the surroundings still contain free CaO, growth of Or regions will continue by reducing the extent of 0, regions. If adequate free CaO is available, 0, region will degenerate into a boundary interface and the belite cluster will be converted into C$. Further, it has been seen that CsS-C2S regions are always separated from CaO regions by a region consisting entirely of C$l + melt in a normal clinker composition(tts).

Burnability and Clinkerization 53

7.8

Non-isothermal

Reaction

Kinetics

Almost all the studies reported earlier on the kinetics of clinkerization reactions were based on isothermal investigations, although it is estimated that only 20% of the reactions in actual practice can be so measured. In studying the rate of clinker formation isothermally, some of the reactions such as the decomposition of calcite with concomitant formation of CzS, are ignored and the actual ones under kiln conditions were not taken into account. The evaluation of non-isothermal data was reportedly attempted(i10Ji6). A more comprehensive study was rnade(te@ with plant raw-mixes. Based on the experimental studies, the course of burning cement raw meals has been summarized by assuming a four-stage model in Table 17. Table 17. Four-stage Model for Non-isothermal reaction Kinetics Stage

Reaction

Mechanism

1

Calcite decomposition

Phase-boundary controlled

570-890

g

2

Solid state reaction I

Phase-boundary controlled

600-800

Not established

3

Solid state reaction II

Diffusion controlled

Solid liquid reaction

Diffusion controlled

4

Temperature range (“C)

1000-1250 1325-1450

Equation (a) =

1-(l-a)“2

g(a) =2

1 - :a - ( 1 -a)2’3 g(a) = 2 I - :a - (1 -@*I3

8. CONCLUSIONS Bumability and reactivity are fundamental concepts of cement making which reflect the variation: in process parameters through mass and energy transfers.

It is necesary to distinguish the concept of burnability from reactivity since this distinction is useful for comparing raw-mixes even with identical composition. As a result, the thermal, chemical and physical behaviours of raw-mixes and clinkers vary unexpectedly from plant to plant. Evaluation of burnability by different methods covers chemical, mineralogical and granulometric behaviour of raw-mix and establishes complicated empirical relations at a certain temperature and time based on free-lime determination. However, cement technologists prefer simple relations based on simple and rapid experiments. To this end, the most rapid and suitable model in laboratory-scale experiments is yet to be selected for general acceptance, a model which could be equally applicable for plant raw-mixes.

54 Progress in Cement and Concrete The reaction sequence in a cement rotary kiln is a complicated phenomenon. The appearance and disappearance of a large number of intermediate phases at early stages make the system unpredictable and unreliable due to the lack of kiln data at operating conditions. Moreover, the presence of minor constituents, solid-liquid-gaseous diffusion environment, and the fluctuation and distribution of temperatures in the kiln make the system more typical and ambiguous. These disputed phases are yet to be fully explored. Mathematical modelling of the kiln processes may be right solution to confirm the interstitial phases formed in the defined zones. The kinetics of clinker formation appears impossible to be expressed by common parameters. However, using the most recent models, the rate of clinker formation could be established by means of apparent diffusion and activation energy data which are believed to be still in the experimental stage. Further investigation is necessary with actual. plant raw-mixes, taking into account all possible factors and projecting the individual character to arrive at more general kinetic characteristics and the equation of clinker formation. *Reproduced with permission from Pergarnon Press.

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