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DEPENDENCE OF THE DENSIFICATION ON GRAIN GROWTH AND ON AGGLOMERATION IN
SINTERING OF DOLOMITE
A. Fonseca, J. Vieira, J. Baptista
To cite this version:
A. Fonseca, J. Vieira, J. Baptista. DEPENDENCE OF THE DENSIFICATION ON GRAIN
GROWTH AND ON AGGLOMERATION IN SINTERING OF DOLOMITE. Journal de Physique
Colloques, 1986, 47 (C1), pp.C1-435-C1-440. �10.1051/jphyscol:1986165�. �jpa-00225596�
JOURNAL DE PHYSIQUE
C o l l o q u e C1, suppl6ment a u n 0 2 , Tome 47, f 6 v r i e r 1986 page ci-435
DEPENDENCE OF THE DENSIFICATION ON GRAIN GROWTH AND ON AGGLOMERATION IN SINTERING OF DOLOMITE
A.T. FONSECA, J.M. VIEIRA and J.L. BAPTISTA
D e p a r t a m e n t o d e E n g e n h a r i a C e r d m i c a e d o V i d r o e C e n t r o d e C e r d m i c a e V i d r o (INIC), U n i v e r s i d a d e d e Aveiro, 3800 Aveiro, P o r t u g a l
R6sumC - Le f r i t t a g e 2 1650
-
1750°C clans l ' a i r e t sous vide de dolomite copr6- cipitee calcin6es sous diff6rentes pr6ssions CO a 6tC 6tudi6. On observe selon l e s cas, un a r r & t de l a densification ou &me une d6densifiCation suivi par une rapide r6cupbration. Ce comportement de l a microstructure au cours du f r i t t a g e .Abstract
-
Sintering a t 1650-
1750°C in a i r and i n vacuum of directly precipi- tated dolomite, calcined under different pressures of CO resulted either i n temporary halt in densification or even in a dedensificagion followed l a t e r by a rapid retraction. The densification behaviour i s correlated with the morpho- logy of the calcined dolomite and with microstructural rearrangement during sintering.Dolomite refractories are widely used i n iron- and steelmaking industry, namely i n Europe. Magnesite and dolomite refractories can give the best performances when judi- ciously chosen f o r the different p a r t s of the linings /I/.
The composition of natural rocks used as raw materials f o r the dolomite refractories includes impurities which are detrimental t o the refractoriness of the product.
Sintering of natural dolomites is dominated by the presence of liquid phase. Yet, the degree of direct bonding of the solid grains, the phase distributions and the struc- ture of the solid skeleton are of major importance for the quality of the refractoly.
In the present study pure dolomite i s prepared by direct precipitation. The material prepared by t h i s process reproduces the natural dolomite synthesis /2/ and it may be used f o r the study of solid s t a t e sintering i n a binary system (MgO-CaO) with i t s components mixed a t the atomic level i n the starting materials.
I1
-
EWERIMENTAL PROCEDUREDolomite, MgCa(C03)2 was prepared by direct precipitation from aqueous MgC12, CaC1, and K2C03 solutions, following the method described by G. Baron /2/. Only dolomite was detected by X-ray diffraction of the precipitated material, the phase being identified with the crystallographic tables f o r rhombohedra1 carbonates /3/.
The chemical analysis gave the following composition: 25.5% mole CaO, 24.5%mole MgO, 49.8% mole C02, 0.03% mole Na20 a l l other detected impurities being l e s s than 200ppm each; the corresponding volume fractions of CaO and MgO i n sintered dolomite were
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986165
C 1 - 4 3 6 J O U R N A L DE PHYSIQUE
C, =0.605 and C,=0.395 respectively.
Calcination was carried out-for 5 hours a t different temperatures and CO~pressures:
a) 750W in vacuum (6.65*10 3 ~ a ) , b) 800?C, 2 .66*103pa and c) 850?C, 1. 33*104 Pa
.
a) and c) correspond t o extreme conditions in the decomposition of dolomite /4/. The corresponding products are referred t o as doloma (a), doloma (b) and doloma ( c ) in the following text.
SEM microscopy revealed that precipitated dolomite is composed of well defined,2U5@m wide particles of rhombohedra1 shape and agglomerates of smaller 4 - 6 ~ wide particles.
1 6 5 0 O C
NON MILLED
doloma:
m - [ b l
.C
doloma (c) :@
--0-1700°C AIR&
-A-1650°C AIR. f
- - k - 1 7 0 O 0 C VACUUM46 doloma (b):
-+I 750°C AIR
FIGURE 2
FIGURE I
1
Figure 1. Transition regions (I-IV) i n isothermal sintering of dolomite calcined with different COP pressures. (-) non-milled dolomas. (---) milled dolomas.
Figure 2. Linear dependence of density on sintering time i n region (11) for dolomas (b) and (c).
Calcination a t 750" i n vacuum preserved the original p a r t i c l e shapes. Powders calci- ned a t 850?C, under 1.33*104 Pa of COz were formed by irregularly shaped and s o f t agglomerates. Milled and non-milled dolomas were used i n the sintering study. Milling was done by hand for 15 minutes in agate mortar i n dry nitrogen inside a glove box.
Isothermal sintering of i s o s t a t i c a l l y pressed p e l l e t s with 0.47-0.48 relative green densities was carried out i n a i r a t 16500C, 17000C and 1750'C and i n vacuum
(1.3*10-~ Pa) a t 1700QC, the time for equilibration of temperature being 4 minutes since the p e l l e t was introduced cool i n the furnace.
Fracture surfacesand polished sections were coated with gold for SEM observation.
The grain boundaries were developed by thermal etching f o r 5 minutes a t 14500C in air. Back scattered electrons were used when enhanced phase contrast was intended in SEM.
Densities of the samples were determined with the mercury balance. The theoretical density of dolom was calculated as 3. 46*103Kgm 3 . The average intercepted lenght of a t e s t straight l i n e , L , and n , the number of pores per unity of area, were determi- ned on polished sections. Grain size was calculated as G=1.5 L. Pore radius, r and pore separation x , were determined by r = P/ (2.1 n)] '/
*
and x = 1.13/NY
where P is the porosity and%
= n/(2 r ) i s the number of pores per unity of voywme.I11
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RESULTS (1) Densif icationThe densification curves corresponding t o isothermal sintering of doloma(b)at1750?C and of doloma (c) a t 1650-17500C show characteristic transitions which divide them into four distinctive regions, referred t o as I-IV in figure lb)
.
Region (I) corres- ponds t o a r a p i d densification and it i s common t o every densification curve. Region(11) is clearly observed f o r curves A,B,D,G i n figure 1. Density i s approximately linear with tin^ i n t h i s region (11), figure 2. Densification rates are one order of magnitude lower than i n the preceeding region (I). Sintering temperature, sintering atmosphere and the calcination conditions remarkably changed the extent of region (11); however they have only small effects on the densification rates of t h i s region, curves B,D,G and N i n figure 2. The dedensification rates i n region (111) are similar t o the corresponding densification rates iq region (11). Curves C and E i n figure 1 show a slowing down in the densification rate instead of dedensification. Finally, i n region (IV) densification recovers t o f i n a l densities which for dolomas (b) and (c) a t 1700-1750PC are a l l close t o the densities achieved with milled powders.
(2) Microstructural evolution
Figure 3. Homgeneous distributions of CaO and MgO (dark) grains i n sintered dolomite.
Sintering 1700?C, air. a) doloma (c), 1 hour; b) doloma (a), 2 hours, f u l l y dense microstructure of a 4 0 - 5 0 ~ wide agglomerate.
Milled dolomas (b) and (c) and doloma (a) with and without milling resulted in homo- geneous distributions of MgO and CaO grains in sintered samples, figure 3. Phase segregation is reported i n figure 4a) and b)
,
f o r non-milled doloma (c) : rows withC1-438 JOURNAL DE PHYSIQUE
five t o ten contacting grains of MgO are observed. Such long rows can hardly be observed i n figure 4c) corresponding t o the end of the dedensification region (111)
,
the phase distribution being more homogeneous. In addition t o large residual pores, new pores which are not seen i n figure 4b) are observed a t grain corners i n figure 4c). These new pores appeared a t the onset of region (111).
Particle coarsening and grain growth are observed during a l l stages of sintering.
Grain growth and pore growth follow the law G = Ktm, the values of m f o r milled do- loma (c) sintered a t 17000C being m = 0.34, m = 0.31 and m = 0.23 f o r CaO and MgO grain sizes and f o r pore size respectively. The MgO and CaO grain sizes follow the same kinetics of growth. Porosity i s composed of residual interagglomerate pores, the average pore separation being three t o five times the grain sizes.
Figure 4. Microstructural evolution during sintering of non-milled doloma (c) a t 1700%: a) a i r , 30 min. ; b) a i r , 70 min. ; c) a i r , 130 min. ; d) vacuum sintered, 40 min. MgO-dark grains.
IV
-
DISCUSSIONTransient effects such as temporary h a l t i n densification, dedensification followed l a t e r by shrinkage have been reported i n sintering of multicomponent systems /5-9/.
These phenomena were explained as resulting from the Kirkendal-Harthley effect /5, 7-9/ i n multicomponent systems o r from swelling due t o entrapped gases and from p a r t i c l e rearrangement in single and multicomponent systems /6,10,11/.
The mutual s o l u b i l i t i e s of MgO and CaO a t 17009C are low: 3 wt% f o r the solubility of MgO i n CaO and 2 wt% f o r the CaO i n MgO /12/. By using Pine's equation /8/ f o r the shrinkage A l / l , of an ideal mixture of insoluble components with equally sized particles, the shrinkage rate for dolomite becomes:
where h,
nc
andn,
are the relative shrinkages of the &I@-MgO, CaO-CaO and MgO-CaO contacEs respectively. From early reported sintering of MgO-CaO mixtures a t1400-1500QC/6/, dn, /dt i n equation (2) is expected t o be much lower than d b / d t and d n /dt f o r the unconstrained contacts. I f , due t o local heterogeneities of phase d?fstributions, figures 4a) and b), the MgO-MgO and CaO-CaO contacts become completely sintered a t the end of region (I) while shrinkage is s t i l l proceeding a t
the MgO-CaO contacts, an important decrease of the densification rate results from equation (1). However, equation (1) does not account f o r interactions between nei&
bour contacts i n planar and tridimensional arrays of particles. The intensity of the interactions being maximal when unlike contacts are close together /13/ milled dolomas should s i n t e r b e t t e r and without transitions, curves J and L i n figure lb).
During the decomposition under high C02 pressures, MgO forms f i r s t /4/ and it beco- m s separated from the well crystallized CaCO, domains, the phase segregation being transmited to the corresponding dolomas
(0)
and (c)). The interfacial energy i n the MgO-CaO contacts i s appreciably l e s s than those in the MgO-MgO and CaO-CaO contacts /6,14/. Therefore the rows of MgO grains reported above are thermodynami- cally unstable, as they correspond t o an excess of specific surface area of the MgO-MgO interface i n comparison with an homogeneous distribution of MgO and CaO grains. Domain densification during sintering /11/ enabled not only grain growth but also phase rearrangement.The precise mechanism of phase rearrangement and the way it resulted i n dedensifica- tion, region ( I I I ) , is not y e t f u l l y understood. C o n , H20 and related gaseous species are rather tightly bound t o the MgU and CaO oxides, outgassing temperatures up t o 1800-2200?C having been reported 115-17/. Poor degasification during the heating up stage /16/, entrappment of insoluble gases can produce high internal pressures i n closed pores in sintering. In identical conditions, milled dolomas (b) and (c) were more rapidly sintered t o the close pore stage than the non-milled ones, figure l b ) ; yet sweeling is not observed i n the milled dolomas. Rayleigh-like i n s t a b i l i t y /11/
of the largest MgO rows, dragging forces resulting from grain growth may cause ope- ning of the MgO-MgO contacts and shortening of the rows. Either, rearrangement ten- sions produce creep-like cavitation a t grain comers, or the intensive transport of MgO and CaO in opposed directions along the MgO-CaO interface turn operative Kirken- dal-like effects /5,7-9/. A t 1700QC the l a t t i c e selfdiffusion coefficients in the oxides are: D=3. 8.10-15m2 s-' for the Mg/18/ and D=4.9*10-'" m2 s-' f o r the Ca/19/;
being D=7.9*10-Ism2 s-' f o r diffusion of Ca i n Mg0/18/. The s c a t t e r of published values f o r selfdiffusion coefficier~ts of MgO is i t s e l f wider than one order of magnitude a t 1 7 W " /20/. Unequal d i f f u s i v i t i e s of the cations can e x i s t i n sintering of dolomite. A s some degree of mutual solubity is also present, the Kirkendal-Har- thley e f f e c t can not be ruled out. SEM microscopy revealed t h a t , having achieved microstructural homogeneity, cavitation o r Kirkendal-like effects cease and recently f o m d pores are rapidly healed, region
(m .
I t is shown that the transitions in sintering behaviour of pure, non-milled dolomas calcined under high C02 pressures resulted from intensive microstructural rearrange- m n t due t o heterogeneities i n phase distributions of the calcined powders.
ACKNOWLEDGMENTS
To Eng? Carlos Sg of the ONW (Universidade do Porto) f o r h i s assistance with the SEM.
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