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Analysis of magnesia chrome refractories weared in a rotary cement kiln
Cement
kiln magnesia
ANALYSIS
chrome
refractories
Ann. Chim.
Sci. Mat, 1998,23,
OF MAGNESIA CHROME RJWRACTORIES IN A ROTARY CEMENT KILN
Z. QOTAIBI,
A. DIOURI,
A. BOUMARI,
M. TAIBI’,
pp. 169-172
WEARED
J. ARIDE’
Laboratoire de Chimie du Solide Appliquee, IAF 501, Faculti des Sciences, Universiti Mohammed Avenue Ibn Batouta, B.P 1014 R.P, Rabat, MAROC. * Laboratoire de Physico-Chimie des MatBriaux, LAF 502, E.N.S. Takaddoum, Rabat, MAROC.
V,
Summary : Magnesia chrome bricks are basic refractories which are essentially composed of magnesia and interstitial phases: MgzSi04 and Mg(Al,,,Cr,,,)O,. These refractories, used in the clinkering zones lining of moroccan cement kilns, have presented a premature wear. X-ray diffraction, scanning electron microscopy, and thermal expansion were used to characterize the damaged refractories. The calorimetric method shows the presence of toxic hexavalent chromium in the hot face of the refractory. Resume : Analyse des refractaires de magnesie-chrome &grades dans un four a ciment. Ces briques sont constitooes de grains de magnesie MgO et de phases interstitielles Mg,SiO., et Mg(Ali,SCro,5)04. L’etude de la degradation de ces refractaires est men&e en utilisant les techniques d’analyse par diffraction X, microscopic dlectronique a balayage et dilatometrie. Des dosages colorimetriques montrent la formation du chrome hexavalent, nocif pour l’environnement.
1. INTRODUCTION Magnesia chrome refractories have made their way in the cement industry because of their important physical properties (1). However, chemical clinker-brick interactions decrease their performance (2-3). Diouri and al. (4) indicate that the infiltration of cement phases, especially p-Ca2Si04 (CrS, C: CaO, S: Si02) in magnesia spine1 retiactory is responsable of destruction of spine1 MgA1204 in the hot side of bricks. -.-_--I___--Reprints: A Boukhari, L.C. S. A., L%?6des Sciences, B.P 1014 RP, RABAT, MAROC
170
2. Qotaibi
et al.
In the case of magnesia chrome refractories, the oxidizing conditions, at certain stages of clinker manufacturing, may convert trivalent chromium of magnesia chrome into a hexavalent state (5). The main object of this study is to explain the wear of magnesia chrome bricks in clinkering zone of moroccan cement rotary kilns. X-ray diffraction, metallographic analysis, dilatometry technic and calorimetric determination were carried out to characterize the damaged refractory. 2. EXPERIMENTAL
TECHNIQUES
The brick was taken from the clinkering zone of a rotary kiln after 27 days. The original thickness was 200 mm and the remaining one was only 56 mm. Going from the hot to the cold face of a brick, we observe an intrados layer (A) with a grey color and 5mm thickness followed by an intermediate layer(B) with brown color and 8 mm thickness. The layer C is the rest of the brick which reaches the extrados limit. The X-ray diffraction patterns were carried out on powder samples (hKa, = 1.5405A). The determination of water’s soluble hexavalent chromium was studied by calorimetric method (6,7), using a Model varian WNIS spectrophotometer. Density values were measured by the aid of picnometry method at room temperature. Microscopic observations were realised with scanning electronic microscope (JEOL, JSMT 330). The observed samples were obtained after a mechanical polishing. Thermal expansion, between room temperature and 12OO”C, was carried out using an electronic dilatometre (NETZSCH 402 EP) with alumina cell. 3. EXPERIMENTAL
RESULTS
3.1. X-ray diffraction Table 1 shows the identified phases and their corresponding semi-quantitative estimation. In the hot face (A), Mg,SiO, is not identified and the spine1 Mg(Al&r,,,)O, is relatively destroyed. The phases C$,C$ and &A, derived from the clinker, were incorporated into the layer A. TABLE
1 - Observed phases of the fresh brick (N), the used refractory
layers (A-C) and clinker (CK).
h&S = MgzSiOd , C,S = (CaO)$iOz , CIA = (CaO)&l& ,CZS = (CaOhSiO2 = (Ca0)4(A1201)(Fe203) , *++ strong, +++ medium, ++ low, f very low.
C&F
3.2. Calorimetric
analysis
Average values of hexavalent chromium content, identified in the clinker and in the brick before and after use are given in table 2. It shows the formation of a hexavalent chromium in the layer A and in the clinker.
Cement
TABLE
kiln magnesia
chrome
refractories
2 - Contents of Cr”’ (mgikg) SAMPLES
171
in brick N, in layers (A,B and C) and in the clinker (CK). N
A
B
C
CK
0
164
0
0
1.2
Contents of Cr”’ (mg/kg) 3.3. Density measurments Table 3 shows density experimental values of weared brick layers, compared and constructor values. This results indicate the compatness of the damaged bricks. TABLE
to non used brick
3 - Density values of magnesia chrome bricks before and after use. Fresh brick(N)
SAMPLES
Manufacm
Density
2.80-2.88
3.4. Metallographic
Damaged brick
Experimental
A
B
C
2.81
3.89
3.87
3.27
observations
Figure 1 shows two micrographs of layer A of a damaged brick. Cubic-shaped sylvine KC1 (8) and needle-shaped belite Ca*SiO., (9) are infiltrated from clinker via microcracks of the refractory.
FIG. 1 - Micrographs
of layer A; a :Ca,SiO, , b : MgO , c : KC1 , + : Crack
The micrograph of layer B (figure 2) shows a long crack near a periclase grain. Figure 3 indicates a microstructure of layer C. Some sub-layers appear at periclase grain surface. These layers can derive from mechanical stresses.
FIG. 2 - Micrograph 3.5. Dilatometry
of layer B
FIG. 3 - Micrograph
of layer C
observations
Figure 4 gives thermal expansion more than the external layers, B and C.
curves of fresh and damaged bricks. The hot layer was dilated
172
FIG.4 - Dilatometric
Z. Qotaibi
et al.
curves of fresh and used brick
The values of a = (L/L,/(T-T,)), between room temperature and 1000°C of the parts of brick A, B and C, are respectively 13.5 1 Oe6, 10.2 10e6 and 9.7 10“ “Cl. These layers have presented a hysteresis effect, which was widened from A to C and have a behavior different from that of the fresh. 4. CONCLUSION The destruction of magnesia chrome bricks in the clinkering zone lining of rotary cement kilns was characterized by 1) The disappearance of Mg$iO, and spine1 Mg(A1,,SCro,5)0, in the intrados face (A), .2) the penetration of clinker phases Ca$iO,, Ca,SiO, and Ca3AlZ06 through the hot side (A) of the refracrory, 3) the formation of hexavalent chromium in the clinker exposed layer (A) of brick and the infiltration of sylvine phase KC1 into the hot face (A). The dilatation of the hot layer (A) is greater than that of the other layers (B and C) which shows progressive wide hysteresis from intrados to extrados face of the used refractory. The different constitutions of analyzed A, B and C layers and their variant dilatometric behaviours can cause some constraints into the refractory bricks and constitute a source of destruction. 5. REFERENCES (1) BARTHA (P.), The properties of periclase spine1 brick and its service stresses in rotary cement kilns, lnterceram, 1984, 33, 15. (2) FREYBURG (E.), KIESTER (J.), Interactions entre le clinker a ciment Portland et la maconnerie refractaire darts les fours rotatifs, Zement kalk Rips, 1985, 6, 393. (3) WAJDOWICZ (A.), MOSCI (R.), ORMELAS (P.), MASHADO (L.), LANA (S.L.B), General wear mechanisms of basic refractories near the burning zone and the transition zone of rotary cement kilns, lnterceram, 1984, 33, 44 . (4) DIOURI (A.), OUICHOU (L.), BOUKHARl (A.), Interaction charge-brique rcfractaire en magnesie spinelle dans un four industriel marocain, J. Chim. phys, 1991, 3 2341. (5) MUNCHBERG (W.), OPITZ (D.), STRADTMANN (J.), The behavior of dolomite bricks in rotary cement kilns, lnterceram, 1984, 33, 33. (6) CHARLOT (G.), BESIER (D.), Analyse quantitative minerale, Masson and Cie, Paris, 1955. (7) FRIAS (M.), SANCHEZDE DE ROJAS (M.1) , GARClAS (N.), LUXAN (M.P.), contribution of toxic elements : hexavalent chromium materials used in the manifacture of cements, Cement and Concrete Research, 1994, 24 , 533. (8) ZEDRICEK (W.), Produits refractaires, Radex international, Paris, 1984. (9) HEIDELBERG (H, X.), Development of burning technology in the cement industry and demands of refractory lining, lnterceram, 1983, 6, 7.
kiln magnesia
ANALYSIS
chrome
refractories
Ann. Chim.
Sci. Mat, 1998,23,
OF MAGNESIA CHROME RJWRACTORIES IN A ROTARY CEMENT KILN
Z. QOTAIBI,
A. DIOURI,
A. BOUMARI,
M. TAIBI’,
pp. 169-172
WEARED
J. ARIDE’
Laboratoire de Chimie du Solide Appliquee, IAF 501, Faculti des Sciences, Universiti Mohammed Avenue Ibn Batouta, B.P 1014 R.P, Rabat, MAROC. * Laboratoire de Physico-Chimie des MatBriaux, LAF 502, E.N.S. Takaddoum, Rabat, MAROC.
V,
Summary : Magnesia chrome bricks are basic refractories which are essentially composed of magnesia and interstitial phases: MgzSi04 and Mg(Al,,,Cr,,,)O,. These refractories, used in the clinkering zones lining of moroccan cement kilns, have presented a premature wear. X-ray diffraction, scanning electron microscopy, and thermal expansion were used to characterize the damaged refractories. The calorimetric method shows the presence of toxic hexavalent chromium in the hot face of the refractory. Resume : Analyse des refractaires de magnesie-chrome &grades dans un four a ciment. Ces briques sont constitooes de grains de magnesie MgO et de phases interstitielles Mg,SiO., et Mg(Ali,SCro,5)04. L’etude de la degradation de ces refractaires est men&e en utilisant les techniques d’analyse par diffraction X, microscopic dlectronique a balayage et dilatometrie. Des dosages colorimetriques montrent la formation du chrome hexavalent, nocif pour l’environnement.
1. INTRODUCTION Magnesia chrome refractories have made their way in the cement industry because of their important physical properties (1). However, chemical clinker-brick interactions decrease their performance (2-3). Diouri and al. (4) indicate that the infiltration of cement phases, especially p-Ca2Si04 (CrS, C: CaO, S: Si02) in magnesia spine1 retiactory is responsable of destruction of spine1 MgA1204 in the hot side of bricks. -.-_--I___--Reprints: A Boukhari, L.C. S. A., L%?6des Sciences, B.P 1014 RP, RABAT, MAROC
170
2. Qotaibi
et al.
In the case of magnesia chrome refractories, the oxidizing conditions, at certain stages of clinker manufacturing, may convert trivalent chromium of magnesia chrome into a hexavalent state (5). The main object of this study is to explain the wear of magnesia chrome bricks in clinkering zone of moroccan cement rotary kilns. X-ray diffraction, metallographic analysis, dilatometry technic and calorimetric determination were carried out to characterize the damaged refractory. 2. EXPERIMENTAL
TECHNIQUES
The brick was taken from the clinkering zone of a rotary kiln after 27 days. The original thickness was 200 mm and the remaining one was only 56 mm. Going from the hot to the cold face of a brick, we observe an intrados layer (A) with a grey color and 5mm thickness followed by an intermediate layer(B) with brown color and 8 mm thickness. The layer C is the rest of the brick which reaches the extrados limit. The X-ray diffraction patterns were carried out on powder samples (hKa, = 1.5405A). The determination of water’s soluble hexavalent chromium was studied by calorimetric method (6,7), using a Model varian WNIS spectrophotometer. Density values were measured by the aid of picnometry method at room temperature. Microscopic observations were realised with scanning electronic microscope (JEOL, JSMT 330). The observed samples were obtained after a mechanical polishing. Thermal expansion, between room temperature and 12OO”C, was carried out using an electronic dilatometre (NETZSCH 402 EP) with alumina cell. 3. EXPERIMENTAL
RESULTS
3.1. X-ray diffraction Table 1 shows the identified phases and their corresponding semi-quantitative estimation. In the hot face (A), Mg,SiO, is not identified and the spine1 Mg(Al&r,,,)O, is relatively destroyed. The phases C$,C$ and &A, derived from the clinker, were incorporated into the layer A. TABLE
1 - Observed phases of the fresh brick (N), the used refractory
layers (A-C) and clinker (CK).
h&S = MgzSiOd , C,S = (CaO)$iOz , CIA = (CaO)&l& ,CZS = (CaOhSiO2 = (Ca0)4(A1201)(Fe203) , *++ strong, +++ medium, ++ low, f very low.
C&F
3.2. Calorimetric
analysis
Average values of hexavalent chromium content, identified in the clinker and in the brick before and after use are given in table 2. It shows the formation of a hexavalent chromium in the layer A and in the clinker.
Cement
TABLE
kiln magnesia
chrome
refractories
2 - Contents of Cr”’ (mgikg) SAMPLES
171
in brick N, in layers (A,B and C) and in the clinker (CK). N
A
B
C
CK
0
164
0
0
1.2
Contents of Cr”’ (mg/kg) 3.3. Density measurments Table 3 shows density experimental values of weared brick layers, compared and constructor values. This results indicate the compatness of the damaged bricks. TABLE
to non used brick
3 - Density values of magnesia chrome bricks before and after use. Fresh brick(N)
SAMPLES
Manufacm
Density
2.80-2.88
3.4. Metallographic
Damaged brick
Experimental
A
B
C
2.81
3.89
3.87
3.27
observations
Figure 1 shows two micrographs of layer A of a damaged brick. Cubic-shaped sylvine KC1 (8) and needle-shaped belite Ca*SiO., (9) are infiltrated from clinker via microcracks of the refractory.
FIG. 1 - Micrographs
of layer A; a :Ca,SiO, , b : MgO , c : KC1 , + : Crack
The micrograph of layer B (figure 2) shows a long crack near a periclase grain. Figure 3 indicates a microstructure of layer C. Some sub-layers appear at periclase grain surface. These layers can derive from mechanical stresses.
FIG. 2 - Micrograph 3.5. Dilatometry
of layer B
FIG. 3 - Micrograph
of layer C
observations
Figure 4 gives thermal expansion more than the external layers, B and C.
curves of fresh and damaged bricks. The hot layer was dilated
172
FIG.4 - Dilatometric
Z. Qotaibi
et al.
curves of fresh and used brick
The values of a = (L/L,/(T-T,)), between room temperature and 1000°C of the parts of brick A, B and C, are respectively 13.5 1 Oe6, 10.2 10e6 and 9.7 10“ “Cl. These layers have presented a hysteresis effect, which was widened from A to C and have a behavior different from that of the fresh. 4. CONCLUSION The destruction of magnesia chrome bricks in the clinkering zone lining of rotary cement kilns was characterized by 1) The disappearance of Mg$iO, and spine1 Mg(A1,,SCro,5)0, in the intrados face (A), .2) the penetration of clinker phases Ca$iO,, Ca,SiO, and Ca3AlZ06 through the hot side (A) of the refracrory, 3) the formation of hexavalent chromium in the clinker exposed layer (A) of brick and the infiltration of sylvine phase KC1 into the hot face (A). The dilatation of the hot layer (A) is greater than that of the other layers (B and C) which shows progressive wide hysteresis from intrados to extrados face of the used refractory. The different constitutions of analyzed A, B and C layers and their variant dilatometric behaviours can cause some constraints into the refractory bricks and constitute a source of destruction. 5. REFERENCES (1) BARTHA (P.), The properties of periclase spine1 brick and its service stresses in rotary cement kilns, lnterceram, 1984, 33, 15. (2) FREYBURG (E.), KIESTER (J.), Interactions entre le clinker a ciment Portland et la maconnerie refractaire darts les fours rotatifs, Zement kalk Rips, 1985, 6, 393. (3) WAJDOWICZ (A.), MOSCI (R.), ORMELAS (P.), MASHADO (L.), LANA (S.L.B), General wear mechanisms of basic refractories near the burning zone and the transition zone of rotary cement kilns, lnterceram, 1984, 33, 44 . (4) DIOURI (A.), OUICHOU (L.), BOUKHARl (A.), Interaction charge-brique rcfractaire en magnesie spinelle dans un four industriel marocain, J. Chim. phys, 1991, 3 2341. (5) MUNCHBERG (W.), OPITZ (D.), STRADTMANN (J.), The behavior of dolomite bricks in rotary cement kilns, lnterceram, 1984, 33, 33. (6) CHARLOT (G.), BESIER (D.), Analyse quantitative minerale, Masson and Cie, Paris, 1955. (7) FRIAS (M.), SANCHEZDE DE ROJAS (M.1) , GARClAS (N.), LUXAN (M.P.), contribution of toxic elements : hexavalent chromium materials used in the manifacture of cements, Cement and Concrete Research, 1994, 24 , 533. (8) ZEDRICEK (W.), Produits refractaires, Radex international, Paris, 1984. (9) HEIDELBERG (H, X.), Development of burning technology in the cement industry and demands of refractory lining, lnterceram, 1983, 6, 7.