501 5. Characterisation of limestone and lime 5.1 Limestone 5.1.1 Composition The main component of limestone is calcium carbonate (CaCO3), also known as calcite, which is formed by the compaction of the remains of coral animals and plants on the bottoms of oceans. It varies from a soft white substance (chalk) to a very hard substance (marble). Most commercial limestone deposits are a brownish rock. The chemical compositions and bulk densities of typical pure limestones are shown in Table 5-1. The bulk density of most limestones is in the range of 2.45 to 2.85 g/cm 3 . Chemical composition Mas (%) Cretaceous limestone Jurassic limestone Devonian limestone Marble C1 C2 J1 J2 D1 D2 D3 M1 CaO 52.47 54.24 55.70 55.11 54.29 55.41 55.51 55.34 MgO 0.30 0.260 0.190 0.400 0.39 0.43 0.400 0.59 SiO 2 4.68 1.860 0.240 0.340 1.83 0.26 0.100 0.08 Fe 2 O 3 0.24 0.080 0.032 0.090 0.21 0.06 0.010 0.05 Al 2 O 3 0.63 0.27 0.043 0.12 0.08 0.13 0.013 0.01 K 2 O 0.08 0.046 0.007 0.017 0.02 - 0.005 0.005 Na 2 O 0.03 0.041 0.013 0.018 0.01 - 0.013 0.01 BaO 0.01 0.01 0.012 0.011 0.02 - 0.008 0.01 SrO 0.03 0.036 0.00 0.005 0.02 - 0.009 0.01 Mn X O Y 0.03 0.016 0.013 0.024 0.02 0.02 0.011 0.004 SO 3 0.05 0.055 - 0.043 0 - - - Weight loss (CO 2 ) 41.50 42.81 43.51 43.62 43.05 43.78 43.54 43.97 Density (gcm -3 ) 2.51 2.57 2.61 2.68 2.68 2.69 2.70 2.71 Table 5-1: Chemical compositions and bulk densities of typical pure limestones Calcite and Magnesite decompose at high temperatures to CaO (lime) and MgO, respectively, and CO2 (carbon dioxide). The fraction of carbon dioxide is determined by the weight loss of the limestone. Molar mass: CaCO3 100 g/mol; MgO 40.3 g/mol; CaO 56 g/mol; MgCO3 84,3 g/mol; CO2 44 g/mol.
Transcript
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5. Characterisation of limestone and lime 5.1 Limestone 5.1.1 Composition The main component of limestone is calcium carbonate (CaCO3), also known as calcite, which is formed by the compaction of the remains of coral animals and plants on the bottoms of oceans. It varies from a soft white substance (chalk) to a very hard substance (marble). Most commercial limestone deposits are a brownish rock. The chemical compositions and bulk densities of typical pure limestones are shown in Table 5-1. The bulk density of most limestones is in the range of 2.45 to 2.85 g/cm3. Chemical composition Mas (%)
Density (gcm-3) 2.51 2.57 2.61 2.68 2.68 2.69 2.70 2.71 Table 5-1: Chemical compositions and bulk densities of typical pure limestones Calcite and Magnesite decompose at high temperatures to CaO (lime) and MgO, respectively, and CO2 (carbon dioxide). The fraction of carbon dioxide is determined by the weight loss of the limestone. Molar mass: CaCO3 100 g/mol; MgO 40.3 g/mol; CaO 56 g/mol; MgCO3 84,3 g/mol; CO2 44 g/mol.
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Therefore, the max. weight losses of calcium carbonate and magnesium carbonate are 44 % and 52.2 %, respectively. Magnesite occurs in stones as Dolomite CaMg (CO3)2 or CaCO3 � MgCO3. Depending on this fraction different designations are common.
The porosity of limestone varies between 2 % and 8 %. The pore size ranges between 10 nm and 100 nm. In the pores, water can be adsorbed. Therefore, limestones with high porosity lead to decrepation in kilns due to the vaporization of the water. Limestone is crushed and screened to serve a wide variety of applications, such as pH adjustment in water treatment, formulated product filler in masonry cements, ready mix concrete, asphalt, joint compounds, etc. and production of stone blocks. 5.2 Lime production The principal users of lime are: steel (lowering slag melting temperature) ~ 35 % environment (desulfurization, water cleaning) ~ 20 % building ~ 10 % chemistry ~ 8 % roads ~ 10 % paper/pcc ~ 7 % non-ferrous metals ~ 3 % agriculture (soil conditioner) ~ 3 % sugar ~ 4 %
World production of lime grew steadily from just under 60 million tonnes in 1960 to a peak of almost 140 million tonnes in 1989. Published estimates of the global production of quicklime suggest that the total was approximately 117 million tonnes in 2003. The United States and China, each accounting for about 20 million tonnes, or 18% of the world output, were followed by Germany and Japan, with about 7% of the world output. In most EU countries, the lime industry is characterised by small and medium sized companies. Recently a small number of large international companies have gained a considerable market share. Nevertheless, there are still more than 100 companies operating in the EU. With an annual production of around
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20 million tonnes of lime, the EU countries produce about 15% of sales-relevant world lime production. The largest producers are Germany, Italy and France, together accounting for about two thirds of the total volume. Some steel plants have their own kilns with lime production, as in Russia for example. The average energy consumption in shaft kilns amounts to 3600-4900 kJ/kg lime (860-1170 kcal/kg lime). In rotary kilns, the energy consumption is about 30 % to 40 % higher. Therefore, the lime industry is a highly energy-intensive industry with energy costs accounting for up to 50% of the total production cost. In the following paragraphs, the material properties of lime will be discussed. The composition and especially the crystal structure of limestone blocks are relatively inhomogeneous. For analysis and measurements of the material properties, only small samples can be used. As a consequence, the measured values of different samples from one block to another scatter considerably. Therefore, a lot of samples from a limestone rock always have to be researched to get representative average values. It has to be taken into account that the quarry can consist of limestones with different properties, because limestone is a natural product. 5.3 Lime density The bulk density of a lime is shown in Figure 5-1 in dependence on the burning time. The samples were calcinated at 1050 °C and then burnt at constant temperatures of 1100 °C, 1200 °C and 1400 °C. The vertical beams represent the scattering of the results for similar samples from the same origin. It can be seen that the density approximately does not change with time. This observation is valid for a lot of limes from other origins. However, the density slightly increases with the burning temperature. This behavior is shown in Figure 5-2 for some limes of different origins. As a consequence, the sintering of the lime strongly depends on temperature and is nearly independent of time.
Fig. 5-1: Density of lime in dependence on the burning time (Wolter 2012)
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Fig. 5-2: Increasing lime density with burning temperature (Wolter 2012) Figure 5-3 shows measurement of the porosity in dependence on the burning time. As it was the case for the density, the porosity does not change with the burning time. The lime A with a density of 1.5 g/cm3 has a porosity of about 60 %. The lime C with a density of 1.9 g/cm3 has a porosity of about 45 %. The higher the density the lower the porosity is.
Fig. 5-3: Porosity of two different limes (Wolter 2012) 5.4 Internal surface area The internal surface area of a lime is shown in Figure 5-4 in dependence on time for various temperatures. At low temperatures, the surface area slightly decreases with time. At high temperatures, the surface area drops a little bit for a short amount of time and then seems to remain constant. As it was the case for the density, the heat
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treatment has a small influence; however, the temperature has the dominant influence. This is demonstrated in Figure 5-5.
Fig. 5-4: Effects of heat treatment time and temperature on pore surface area
of burnt lime (Turkdogan et al. 1973)
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Fig. 5-5: Pore surface area of burnt lime (extrapolated to zero heat treatment time) compared with that of porous iron formed by H2-reduction of porous hematite ore (Turkdogan et al. 1973)
Figure 5-6 shows the mean pore diameter of lime samples from different origins. The values are in the range of 100 to 1000 nm and increase with temperature. The shrinkage of the internal surface area with the temperature results in larger pore sizes.
Fig. 5-6: The influence of the oxide shell temperature on the mean pore
diameter (the composition of the limestones is given in Table 5-1) (Kainer et al. 1986)
5.5 Lime quality 5.5.1 Reactivity The most important quality criteria of lime is its reactivity. The lower the decomposition temperature is held during the decomposition of limestone, the higher the lime reactivity will be. In practice, the lime reactivity is detected by the rate of temperature increase of the water-lime-slurry after 150 g of lime powder with a grain
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size of 0-3 mm is dosed into 600 ml of distilled water at 20°C. From the slaking-curve, which displays the temperature increase of the slurry due to the hydration reaction of lime, a parameter t60 can be obtained, which is defined as the amount of time it takes the slurry temperature to increase from 20°C to 60°C (see DIN EN 459-2 2002). When t60 is shorter than 2 min, the lime is said to be soft-burnt. In Figure 5-7, measurements are shown for three different limes. From the index t60, the three limes with different reactivities are differentiated as soft, middle and hard-burnt.
Fig. 5-7: Measurement of lime reactivity, (Modified from T. Schwertmann
2007) The t60-value correlates with a specific surface area of lime (for example BET surface area), or the porosity of the lime. The higher the temperature at the end of burning, the smaller the specific surface area and the porosity will be; hence, the t60-value will be larger. This is decided by the development of the crystal structure or the sintering effect in the lime. Under a Scanning Electronic Microscope (SEM), limes of different reactivity have different crystal structures and pores systems, which are shown in Figure 5-8 and Figure 5-9. A soft burnt lime typically has internal surface areas in the range of 2 - 3 m2/g. A hard burnt lime has internal surface areas below 0.8 m2/g. From the t60-value, sometimes a so called R-value is calculated with:
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min
C
t
40
t
2060
t
TR
6060
(5-1)
The R-values of Annular Shaft Kilns are usually about 20, namely t60=2 min, but it happens sometimes as well that the R-value is smaller than 17 or larger than 30. To characterize the lime reactivity in steel plants, a so called index Coarse Grain Titration is used.
Fig. 5-8: REM pictures of lime (Modified from T. Schwertmann 2007), soft,
middle and hard burnt from left to right
Fig. 5-9: REM pictures (Wolter 2012) 5.5.2 Residual CO2 in lime A further measure for the quality is the residual CO2 in the lime. This is the unreacted, non-decomposed calcite CaCO3. The residual CO2 content is described by the mass of un-reacted CO2 relative to the mass of produced lime. It can be easily measured by the weight loss when the lime samples leaving the kiln are heated up again to the decomposition temperature. If MCO2R is the reacted mass of CO2, MCO2
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the initial mass of CO2 in the limestone and MLS the mass of the limestone, the res. CO2 is expressed by
RCOLS
RCOCO2
2
22
MM
MMCO.resy
. (5-2)
The initial mass of the CO2 in the limestone is described by
LSCOCO MyM22
(5-3)
where
2COy is the CO2 contained in the limestone, which is measured by the weight
loss. Values can be seen as an example in Table 5-1. The conversion degree is defined as the mass of the reacted carbon dioxide relative to the carbon dioxide content in limestone:
2
2
CO
RCO
M
M
. (5-4)
From the above equations, the relation between conversion degree and residual CO2 content is obtained as
2
2
CO2
2CO
yCO.resy1
CO.resyy
(5-5)
or
1y
11
y
11
COresy
22 COCO
2
. (5-6)
This correlation is shown in Figure 5-10. For a carbon dioxide content in limestone of 0.42 and a residual CO2 content of 0.01, the conversion degree is 0.986.
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Fig. 5-10: Conversion degree as a function of residual CO2 content for
different limestones 5.6 Specific Heat Capacity 5.6.1 Limestone The real specific heat capacity of four limestones from different origins is shown in Figure 5-11 in dependence on temperature. The value differs with 0.05 kJ/kg/K only a little bit. The specific heat capacities measured with different samples from the same limestones have similar results [Silva et al. 2005]. The differences therewith belong to the kind of limestones. The line in the figure represents the values given in the handbooks of material properties Barin, Knacke [1973] and Landolt, Börnstein [2002]. These values can be considered as average values of the real specific heat capacity of limestones. It can be approximated by
30,0
p K473
KTK/kg/kJ0,1Tc
. (5-7)
This correlation is similar to the approximation of the temperature dependency as for gases. From the equation above, we get the correlation for the mean specific heat capacity as
1T/T
1T/T
30,1
1K/kg/kJ0,1c
0
3/10
pm
. (5-8)
with T0 = 473 K. Its values are shown in Figure 5-11 as a dotted line.
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Fig. 5-11: Real specific heat capacity of limestones from different origins 5.6.2 Lime Samples of the four limestones A, B, C, D were decomposed in an atmosphere of pure nitrogen and then immediately the specific heat capacity of the calcinated lime was measured. From each limestone, three samples were produced and researched. Each test was repeated once. The results are presented in Figures 5-12a until d (Silva et al. 2010). The samples from each kind of lime show individual profiles. The deviation in the values of the three samples from one kind of lime is relatively high. The repeated tests always match with the first test. Therewith, it can be concluded from these results that the properties within the samples of one kind of limestone scatter considerably. This was also the case in the measured densities and porosities. In the following, the profiles of the specific heat capacities will be discussed.
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Fig. 5-12a: Dispersion of the measured data of the specific heat capacity
among samples of lime A The specific heat capacities of lime A shown in Figure 5-12a are nearly independent of temperature. Sample 1 has a peak at 400 °C, which suggests an endothermic reaction. This reaction may be a decomposition of formed calcium hydroxide Ca(OH)2. The valley at 1200 °C may be an exothermic sintering reaction with including minerals. The specific heat capacities of samples 1 and 3 are similar. However, the values of sample 2, which are at about 1,2 kJ/kg/K, are much higher than all other values given for lime. These high values cannot be explained.
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Fig. 5-12b: Measured data of the specific heat capacity for samples of lime B The specific heat capacities of lime B are shown in Figure 5-12b. The values slightly increase until 500 °C and then remain constant (samples 1, 3) or decrease continuously (sample 2). All profiles have a peak at 1050 °C. The endothermic reaction is unknown. Sample 2 may have sintering reactions at high temperatures which cause the decease of the profile.
Fig. 5-12c: Measured data of the specific heat capacity for samples of lime C
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Fig. 5-12d: Measured specific heat capacity for samples of lime D The profiles of sample C in Figure 5-12c are slightly wavy. The profiles of samples 1 and 2 are similar and continuously increase with temperature. The values for sample 3 remain constant above 800 °C and then sharply decrease starting at 1200 °C, which may again be caused by a sintering process. The profiles of lime D presented in Figure 5-12d increase with temperature until 1000 °C and show a significant peak at 350 °C and a weak peak at 800 °C. Above 1200 °C, the values seem to decrease sharply, as it was the case for most of the other lime samples. From the four figures above it can be seen that there is a considerably deviation in the values for one kind of lime. This was not the case for the corresponding limestone. Here, all samples from the same limestone give nearly identical results. As a consequence, the structure of the produced lime must be much more inhomogeneous than the limestone. Nevertheless, the values of the specific heat capacity of the four limes are a little bit different. Figure 5-13 presents the average values of the three samples from one lime. The range is from 0,8 to 0,95 kJ/kg/K. The values are therewith lower than those for limestone and have a much lower increase with temperature. The values from the handbook of material properties are also included in the figure with a cross. The values continuously increase with temperature and are slightly too high. This behavior of the heat capacities influences the temperature dependency of the decomposition enthalpy, which will be discussed in the following chapter.
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Fig. 5-13: Average values of the real specific heat capacity of lime from
different origins 5.7 Reaction enthalpy 5.7.1 Enthalpy at standard temperature Table 5-2 shows reported values of measurements of the molar enthalpy of the formation for calcium carbonate at 298 K. They vary from 170 to 182 kJ/mol by 7 %. The question arises whether there is only one correct value of enthalpy, meaning that the difference is caused by a measurement error, or if the enthalpy of decomposition varies with the origin of the stone. Chai et al. [19] extensively investigated this value and carried out a comparative evaluation with several reported values and measuring methods. Resulting from this, a value of 178.0 kJ/mol has been recommended as a working basis. This value fits best with the enthalpy of formation from other reactions with limestone.
Author Enthalpy (kJ/mol)
Chai et al. (1992) [19] -178.31.2
Stanmore et al. (2005) [88] -182.1
Helgeson et al. (1978) [32] -179.6
Berman et al. (1998) [8] -178.2
Holland et al. (1990) [38] -180.01.1
Robie et al. (1978) [80] -178.81.0
Watkinson et al. (1982) [92] -170.0
Schwiete et al. (1956) [83] -176.7
Recommended -178.0
Table 5-2: Enthalpies of formation for CaCO3 at 0 °C
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The molar enthalpy of
mol/kJ1178h~ (5-9)
is converted into the enthalpy related to the mass component using its molecular weight
This enthalpy at the standard temperature of 0 °C must be used for energy balances of kilns. This is so that the enthalpies of the flue gas, the discharged lime and the feedstock are also related to the temperature 0 °C. The decomposition enthalpy depends on temperature. As a consequence, for the calculation of the decomposition time the value of the enthalpy at a decomposition temperature of approximately 900 °C must be used. This will be explained in the following. 5.7.2 Enthalpy at decomposition temperature The temperature dependency of the reaction enthalpy is caused by the difference of the specific heat capacities of educts and products. This relation can be obtained by applying Hess’s law. The energy consumption must be independent of the decomposition process. If the limestone is decomposed at T0, then the products CaO and CO2 have to be heated up to the temperature T. If the limestone is decomposed at the temperature T, then CaCO3 has to be heated up beforehand from T0 to T. This results in
Herein is: yCaO the mass fraction of CaO in CaCO3 including the other minerals, yCO2 the mass fraction of CO2 in CaCO3 (1-yCaO), cCaO the mean specific heat capacity of CaO between T0 and T, cPCO2 the mean specific heat capacity of CO2 between T0 and T, and cCaCO3 the mean specific hat capacity of CaCO3 between T0 and T. From the above equation, it follows that the difference between the reaction enthalpies at different temperatures is:
0CaCOpCOCOCaOCaO0CaCOCaCO TTccycyThTh32233
. (5-13)
If in this equation a minimum and a maximum value of the mean specific heat capacities of lime and limestone (0.82, 0.92, 1.10, 1.17 J/g/K) and the mean specific
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heat capacity of CO2 (1.09) is inserted, the result is decomposition enthalpies at 900 °C between 1570 and 1690 J/g CaCO3. These values are lower than the value for 0 °C. In Figure 5-14 measurements are shown of the decomposition enthalpy for the four different limestones considered before (Silva et al. 2010). Two samples of limestone A have nearly identical values, but the value of the third sample is considerably lower and has a somewhat different profile.
Fig. 5-14: Decomposition enthalpy of limestones A, B, C, D in J/g CaCO3
Limestone B has a significantly different profile than the other three limestones. All profiles differ in the height of the peak, which is in the range of 10 to 14. Also in the measurements there is a considerable fluctuation within the samples of one kind of limestone. However, between all kinds of limestone a significant difference in the individual profiles and the mean value of the three samples exists. This mean value
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for the decomposition enthalpy is in the range of 1604 to 1720 J/g CaCO3. From these measurements and the theoretical calculation done before, it can be concluded that the decomposition enthalpy at 900 °C depends on the origin of the limestone. The estimated values are in the range
mol/kJ172157C900h . (5-14) 5.8 Equilibrium pressure The equilibrium pressure, also known as decomposition pressure, can be approximated with the Arrhenius type correlation
00eq T
1
T
1
R
h~
exppp , (5-15)
where p0 is the pressure at the temperature T0, h~ the molar enthalpy at the
temperature T and R the universal gas constant. The molar enthalpy has been discussed before. The temperature T0 can be measured if the limestone samples are decomposed in a pure atmosphere of CO2 at 1 bar ambient pressure. Table 5-3 summarizes measured decomposition temperatures in CO2 at 1 bar of different samples from limestones A, B, C, D and marble (Silva et al. 2010). It can be seen that the temperature scatters within the samples from the same kind of limestone. For limestone B and marble, the values differ by 10 Kelvin. Limestones A, B., C, D have relatively similar decomposition temperatures. However, marble has a decomposition temperature which is 10 to 13 K higher. Table 5-4 depicts average values of the decomposition temperature from a lot of limestones from all over the world. The values are in range of 907 °C to 923 °C. It can be concluded that the decomposition temperature and therewith the equilibrium pressure also depend on the origin of the limestone, as it was the case for the other material properties. Table 5-4 also contains decomposition temperatures of minerals containing CaCO3
(Silva et al. 2010). Cement raw meal has with 907 °C a similar decomposition temperature to limestone. In literature, it is sometimes reported that cement raw meal should have a significantly lower decomposition temperature. This cannot be confirmed. The second decomposition stage of dolomite has a similar decomposition temperature as limestone. The MgO minerals seem to not influence the decomposition pressure. The two clay powders A and B are raw materials used for burning bricks. The CaCO3 content is in the range of 5 to 10 %. These clays seem to have a lower decomposition temperature which is about 60 K lower than that of limestone. The reason is unknown.
Table 5-3: Decomposition temperatures for limestone samples in CO2 at 1 bar Table 5-5 shows decomposition temperatures at 1 bar CO2 that have been reported in literature. The values vary between 892 °C and 910 °C. However, in the extended research of Silva et al. 2005 no value below 900 °C could be detected. Figure 5-15 depicts measured decomposition pressures in dependence of temperature from various authors. As expected, the values differ considerably. Therefore, the values of the pressure have to be dependent on the kind of limestone. The gradient of the measured values gives the decomposition enthalpy. For an average decomposition pressure of 1 bar, the temperature
K1183C910T0
and the molar decomposition enthalpy
mol/kJ164h~
are recommended. With these values, we get from Eq. (5-14)
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barTR
mol/kJ164exp1074.1p 7
eq
.
This correlation is shown in Figure 5-15 with a line.
Material Average Decomposition
Temperature / °C
Limestone A (Brazil) 907 Limestone B (Brazil) 910 Limestone C (Germany) 908 Limestone D (Germany) 908 Chalky Limestone (Italy) 917 Crystalline Limestone (Italy) 917 Limestone (Greece) 923 Limestone (Austria) 919 Limestone (Mexico) 911 Limestone (Oman) 923 Limestone (China) 920 Limestone (Bulgaria) 910 Limestone (Jamaica) 911 Marble 920 Cement raw meal 907 Dolomite 907 Clay Powder Type A 899 Clay Powder Type B 848
Table 5-4: Average decomposition temperatures at 1 bar CO2 of limestones and some materials with a high content of CaCO3
Table 5-5: Decomposition temperature measured in pure CO2 according to various authors
In Figure 5-16 this correlation is shown again with some typical temperatures. The CO2-concentration of the gas in lime shaft kilns is in the range of 20 – 40 Vol % depending on the kind of fuel and the excess air number. This results in starting temperatures of the decomposition of 820 to 840 °C. The CO2 concentration in the atmosphere nowadays is about 390 ppm. This corresponds with an equilibrium temperature of 550 °C. As a consequence, below this temperature lime reacts with the CO2 in the atmosphere to create CaCO3. This process is called recarbonization.
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Fig. 5-15: Equilibrium pressure of limestone
Fig. 5-16: Characteristic values of the equilibrium decomposition pressure of
limestone
T = 531 °C= 390 ppm
0
0
0
0
0
1
10
7 8 9 10 11 12 13
2COX
2COX
T = 806-850 °C= 20-40%(v)
Peq
in b
ar
1/T in 10-4 K-1
T = 910 °Cp = 1 bar
T in °C
101
100
10-1
10-2
10-3
10-4
10-5
5006007008009001000
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5.9 Thermal diffusivity The thermal diffusivity, which is the conductivity divided by the density and the specific heat capacity, will be discussed in this chapter. This material property is not of interest for calculating the decomposition behavior. However, this material property can be relatively easily and exactly measured using a Laser Flash Apparatus (LSA). One side of a disk shaped sample is impinged with a laser impulse and the resulting increase in temperature is measured on the other side with infrared thermography. From the time dependent temperature profile, the thermal diffusivity can be calculated from the Fourier differential equation. Figure 5-17 shows the measured values for three different kinds of limestone (Silva et al. 2008). The values are close together. Also, the reproducibility is high. The Figures 5-18 show the thermal diffusivity of the limes from the four kinds of limestones discussed before. The two samples of lime A have similar values. Also, the reproducibility is high. The three samples of lime B have somewhat different values. Sample 1 has slightly lower values than sample 3. All five samples of lime D have similar values in the range between 600 °C and 1200 °C. However, at low temperatures the values scatter considerably. Lime C has a totally different behavior. Here, the values scatter considerably at all temperatures. Even the reproducibility is low, considering that test 1 and test 2 of the same sample give different results. The average values of the thermal diffusivity are different for all four limes. This will be discussed in the next chapter considering the conductivity.
Fig. 5-17: Thermal diffusivity for different kinds of limestone
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Fig. 5-18a: Thermal diffusivity of limestone A
Fig. 5-18b: Thermal diffusivity of limestone B
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Fig. 5-18c: Thermal diffusivity of limestone C
Fig. 5-18d: Thermal diffusivity of limestone D 5.10 Thermal conductivity The thermal conductivity of different limestones is shown in Figure 5-19. It decreases with temperature, as it is typical for rocks. The values are calculated from the measured values of the thermal diffusivity and the specific heat capacity. All values are similar. They can be approximated with
Km
W
T
K55646,0LS
. (5-16)
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The thermal conductivity of the four limes is shown in Figure 5-20. The values are again calculated with the average values of the thermal diffusivity and the specific heat capacity. The values decrease with temperature. The conductivities of limes C and D are similar. However, limes B and A have considerably lower values. At decomposition temperatures of 900 °C to 1100 °C, the conductivities lie in the range between 0.95 and 0.70 W/m/K. The decomposition time is strongly influenced by the conductivity, as will be shown in the next chapter. Therewith, the decomposition time and the decomposition temperature depend on the kind of limestone. It is remarkable that the conductivity of lime at high temperatures is in the same range than that of limestone. At ambient temperature, the conductivity of limestone, which is 2.5 W/m/K, is much higher than that of lime, which is 1.1 to 1.7 W/m/K. However, at 1000 °C the conductivity of limestone is about 0.9 W/m/K, which is the same value as the two limes C and D.
Fig. 5-19: Thermal conductivity for different kinds of limestone
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Fig. 5-20: Average thermal conductivity of different kinds of lime 5.11 Emissivity The spectral emissivity of a typical lime is depicted in Figure 5-21. At high wavelengths, the spectral emissivity is nearly one, seen here as a black body. The spectral emissivity sharply drops at a value of about 5 μm. The profiles are independent of temperature. This behavior is typical for mineral matter. Fig. 5-21: Spectral emissivity of a pure lime
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The overall emissivity is the spectral emissivity weighted with the intensity of black radiation. Figure 5-22 shows the overall emissivity for three lime samples and one limestone. The values for the limestone and the two pure limes decrease continuously with temperature. Limestone has significantly higher emissivities than lime. The value for white lime drops to 0.6 at high temperatures. The Figure also includes the emissivity of a lime calcinated from a limestone with only 90 % CaCO3. The non-calcite minerals react at higher temperatures and form a layer of slag. Above temperatures of 1300 °C, the slag changes its color to black which results in an increase in the emissivity. Therefore, the emissivity can be a measure to evaluate the reaction behavior of the non-calcite minerals with increasing temperature. Fig. 5-22: Emissivity of a lime 5.12 Thermal expansion An important property of limestone is its thermal expansion because this behavior can lead to a clogging of the packed bed in the kiln. Figure 5.23 shows the thermal expansion for different limestones in dependence on the temperature. It can be seen that the thermal expansion reaches a maximum value between 900 °C and 945 °C. Then the expansion decreases rapidly because of the decomposition. The maximum expansion fluctuates between 1.1 % and 2.7 % with a factor more than two. A connection between the thermal expansion and density, composition or structure could not be found. For many limestones, the maximum value of the thermal expansion is lower the higher the porosity is (Wolter 2012). Fig. 5-23: Thermal expansion of limestones (Wolter 2012)
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5.13 Slag forming At high temperatures, the impurities in the limestone can form a layer of slag on the surface. These liquid layers of different particles can melt together and freeze during the cooling. This results in big blocks which can lead to clogging of the movement of the material. The mean concentration of the impurities can be: SiO2 up to 10 %, Al2O3 up to 8 %, Fe2O3 up to 5 %. In some limestones, the calcite concentration is only in the range of 80 -85 %. The distribution of the impurities is not homogenous. In some small layers, the local concentrations can be very high. Figure 5-24 shows the two phase diagram of CaO and Al2O3. The melting temperature of both pure components is above 2000 °C. However, at a fraction of 50 % an eutectic occurs where the liquidus line falls to 1360 – 1392 °C. Figure 5-25 shows the three phase diagram for the mixture of CaO-Al2O3-SiO2. It can be seen that some mixtures occur with liquidus temperatures in the range of 1200 °C to 1300 °C. In Figure 5-26, the three phase diagram of the mixture CaO-Fe2O3-SiO2 is shown. In this system, a mixture with the liquidus temperature of only 1195 °C can occur. As a consequence, limestones with a high amount of impurities need relatively low calcination temperatures to prevent a gluing of stones with the risk of clogging.
Fig. 5-26: Phase diagram CaO-Fe2O3-SiO2 (Slag atlas 1981) 5.14 Conclusion of this chapter The specific heat capacity values of limestone increase with temperature, and
do not differ greatly from those values reported by some authors [6], [63]. They are not significantly influenced by the origin of the stone. However, the specific heat capacity of lime is influenced by the origin of the limestone. Each kind of lime shows an individual profile of the specific heat capacity with temperature. The specific heat capacity of the same origin may differ significantly. The values of each kind may fluctuate up to 10%. Nevertheless, repeated measurements of the same sample give similar results. The lime specific heat capacity is nearly independent of temperature.
It was experimentally demonstrated that the decomposition enthalpy varies from limestone to limestone and among samples of the same nature. Considering this fact, it can be expected, according to the correlation of Arrhenius, that the decomposition pressure and therewith the decomposition temperature of limestone varies.
532
The starting decomposition temperature of several limestone rocks was experimentally determined, and a decomposition temperature range between 907 °C and 920 °C was established, confirming that an unique temperature does not exist, as it had been previously stated. This result remains the same whether using a solid disk or powder for the tests.
From experimental data of the specific heat capacity and thermal diffusivity of limestone and lime of different origins, the thermal conductivity was determined. As well as the specific heat capacity of limestone, cp, its thermal diffusivity is definitely not influenced by the origin of the stone and therefore, neither is the thermal conductivity.
The thermal diffusivity of lime decreases with temperature and varies according to the origin of the stone. Thus, the thermal conductivity of lime varies according to the origin of the rock.
These findings about the thermal properties of limestone and lime constitute for the lime industry a new and better understanding of the diversity in the reactivity of the produced lime.
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