Effect of rare-earth oxide concentrate on reaction, densification and slag resistance of Al2O3SiO2 ceramic refractories

Ceramics hlternational 20 (1994) 31-37

Effect of R a r e - E a r t h Oxide C o n c e n t r a t e on Reaction, D e n s i f i c a t i o n and Slag Resistance of AI203-SiO 2 C e r a m i c Refractories Chen-Feng Chan & Yung-Chao Ko China Steel Corporation, P.O. Box 47-29, Hsiao Kang, Kaohsiung 81233, Taiwan (Received 5 January 1993: accepted I March 1993)

Abstract: Ceramic bodies of mullite composition were prepared from a mixture of calcined alumina and raw kaolin with the addition of bastnasite (rare-earth oxide concentrate) at up to 3% wt and fired at 1400-1650~C. The bastnasite addition effectively enhances mullite formation but the effect diminishes rapidly with increasing temperature. The Al_,O3-rich composition is characteristic of the mullite. The ceramic green compacts with the bastnasite addition can only approx, reach 96% theoretical density at best when heated at 1550-1650°C. The pores in the fired bodies with the bastnasite addition are smaller and uniformly distributed: localized, larger pores are found in the fired bodies without the addition. The slag resistance of the refractories increases slightly with an increasing amount of bastnasite added to the bonding matrix. Bastnasite is not an effective additive for enhancing the slag resistance of steel-plant refractories. The peak temperature in DTA and the valley temperature in the dilatometric analysis, are indicative of mullite formation; the former was found to be 1412"C without bastnasite addition and 1368~C with 3% wt bastnasite addition while the latter was found to be 1420~C without bastnasite addition and 1350~C with 3% wt bastnasite addition.

1 INTRODUCTION

up to its solubility limit 3 but it hinders sintering and drastically increases porosity and mean pore size above its solubility limit. 4 CaO, Na20, and K 2 0 are effective mineralizers at low concentrations (< 1% wt). 3 The addition of CeO2 to bauxite enhances the resistance to attack of the aluminosilicate refractories for molten 7075 aluminum alloy. 5 The addition of small amounts of rare-earth oxides to the aluminosilicates also effectively reduces attack by molten aluminum alloy; bastnasite, a rare-earth concentrate, was found to be a more effective additive at low concentration (< 15% wt) than either CeO2

Mullite is one of the most important materials for industrial ceramic products. It has superior properties of low thermal expansion, low dielectric constant, low creep rate and high chemical stability. Cheap raw materials can be used to produce highpurity mullite. L Controlled amounts of impurities are added as sintering aids, grain-growth inhibitors and mullite-toughening agents. Many contradictory observations on the effects of the additives fill the literature. The impurities frequently added to the A1203-SiO 2 mixture are Fe203, TiO2, CaO, N a 2 0 and K20. Fe203 is reported to promote mullitization and grain growth, 2"3 TiO2 reportedly enhances sintering and mullitization and inhibits grain growth

or La20 3 alone. 6

The addition of bastnasite or rare-earth oxides to the aluminosilicate refractories for use in a steel plant has not been found in the literature. The 31

Ceramics International 0272-8842/94/$07.00 O 1994 Elsevier Science Limited, England and Techna S.r.I. Printed in Great Britain

32

C.-F. Chart, Y.-C. Ko Table 1. Chemical analysis of raw materials Material

Composition (% wt)

Kaolin Calcined alumina China diaspore

AI20 =

Si02

36"6 99.5 85"2

45'0 0.01 7.2

Fe203

TiO=

Na=O

K=O

LOI

0"85 0.01 2.23

0.27 -3.95

0.01 0.42 0.51

0'8 ---

14'2 ---

Table 2. Chemical analysis of bastnasite Material

Bastnasite

Composition (% wt) CeO 2

La203

Nd203

PrnO~

CaO

BaO

SiO 2

SO,2

F-

44

29

10

4

1.23

1.80

1.70

1.50

3.90

purpose of the present work was to study the effect of bastnasite on reaction and densification of the kaolin-alumina mixture, and the slag resistance of the aluminosilicate refractories for use in a torpedo ladle in a steel plant. 2 EXPERIMENTAL

PROCEDURES

2. 1 Materials Tile chemical composition of the raw materials is given in Tables 1 and 2; the mean particle size was 3"24 l~m for the kaolin and 1 l~m for the alumina. " 2.2 Procedure A ball mill (2 litres) was employed for mixing and milling. The balls were 10 and 5 mm in diameter, respectively, and the weight ratio of 10 to 5 mm balls was 2:1. The inner lining of the drum and the balls were of alumina. The kaolin and alumina powders along with the additives, in deionized water with a little isopropyl alcohol (approx. 1% wt of water) added as dispersant, were mixed in the ball mill at 30rpm for 4h in accordance with the weight ratio of powders/water/ball = 2:3:6. Powders containing 72 or 6 5 % w t AIzO 3 were prepared by mixing raw kaolin, calcined alumina and 0 - 1 5 % wt bastnasite, a rare-earth oxide concentrate. After drying, the powders were passed through a 0.147mm opening. Discs, 3 0 m m in diameter and 10g in weight, were formed by die-pressing dry powder under a load of 120 MPa. The density of the green compact was approx. 66% of theoretical density. After drying at I10~C for 16 h, these discs were fired between 1400 and 1650°C for the quantitative determination ofmullite content and to

study the effect of bastnasite addition on mullite formation, densification and morphology of the fired bodies. The heating rates were 20~C/min below 1 4 0 0 C and 5 C/rain above 1400:C. The mullite content of the fired bodies was determined by the internal-standard method using fluorite as an internal standard. 7 The reflections used in this study are listed in Table 3. The crucibles, 6 0 m m x 60 m m x 6 0 m m cubes with a cylindrical hole of 3 0 m m diameter and 3 0 m m depth in the center, were formed under a pressure of 45 MPa, dried at 110"C for 16 h and then fired at 1300"C or 1450~C for 3h. The bonding matrix consisted of the powder containing 65% wt A120 3 and 0 - 1 5 % w t bastnasite. The aggregates were 8 5 % w t A]20 3 China diaspore. The mix grading for the crucible is listed in Table 4, and the composition of the slag is given in Table 5. The crucibles which had been fired at 1300°C for 3 h were filled with 30g of slag and then retired at 1500~C for another 3 h. The crucibles which had been fired at 1 4 5 0 C for 3 h were filled with 30 g slag and retired at 150OC for 3 h for the 1st stage, and then refilled with 25 g slag and retired at 1500°C for another 3 h for the second stage. These crucibles were cooled naturally in the furnace and cut at the center. The relative wear rate was defined as 104 x the weight of the paper Table 3. X-ray reflections used in quantitative determination of mineralogical composition Mineral

Corundum M ullite Cristobalite Fluorite al A = 0 ' I

nm.

Reflection index

20 value (deg)

d (~)a

(11 3) (121 ) (101 ) (111 )

43.37 40.88 21.93 28.29

2.085 2.206 4.050 3.151

Mullite formation in presence of bastnasite

33

Table 4. Mix grading of refractories

1412

Grain size (mm)

Amount (%wt)

3'36-1.41 1.41 -0.42 0'42-0'25 0.25-0.074 < 0.074 ~31~m

40 14 7 10 14 15

Exo

1

•

Table 5. Chemical analysis of slag Constituent

(% wt)

CaO SiO= AI203 Fe203 MgO

43.3 33.3 13.8 1.2 8.1

K=O

0'1

Na20

0.1

chip cut out from the wear area on the cross-section of the crucible cut at the center. 3 RESULTS

AND

DISCUSSION

Fig. I. DTA of mullite composition mixes with 0 and 3% wt bastnasite added. Heating rate: 15 C/min. liquid phase to react with corundum, s This is affected by the viscosity, which is a function of temperature and liquid composition. There is little doubt that the rate of mullite formation depends on the rate of silica diffusion, which increases with a decrease in the viscosity of the liquid phase. Rareearth oxides, C a O and BaO are network

3. 1 Formation of mullite 100

Figure 1 shows that in differential thermal analysis (DTA) the second peak is at 1368°C for the mix with 3% wt bastnasite addition and at 1412°C for the mix without bastnasite addition. X-ray diffraction (XRD) revealed that the second peak is related to the mullite formation temperature. Apparently, addition of bastnasite to the mullite composition mix is effective in lowering the mullite formation temperature, Figure 2 shows that mullite formation increases with increasing temperature and amount of bastnasite added, but the effect of the bastnasite addition diminishes rapidly as the temperature rises. It is well established that mullite formation is brought about by silica being transferred from clay through the

a0

7 i

a0 70

®

,~

60 so 40

,

I 0

I

I

I

1

2

3

Bastnasite

content

0 1 2 3

+4oo "C

A •

I , ~ o 'C

1See l 4

(w1%)

Fig. 2. Effcctof bastnasite content ofceramic bodies containing 72%wt AI203 on mullite Ibrmation after firing at various temperatures for 1h.

Table 6. Chemical analysis of the mullite obtained by leaching the fired bodies with 0-3% w t bastnasite added Bastnasite added (wt%) a

o

Composition (% wt)

AI=O3

SiO=

TiO=

Fe203

K=O

Na=O

71 '1 76'1 76-8 76"2

27"4 19'7 17"9 17'0

0"18 0"14 0'13 0"12

0"56 0'55 0"52 0"52

0"047 0"035 0"032 0-034

0"2 0"13 0'18 0"15

a0, 1,2, and 3 were related to the 0-3% wt bastnasite added to ceramic bodies from which mullite was leached for the analysis.

34

C.-F. Chan, Y.-C. Ko 100 1600~

-2 A

95

*~ -3. w c

~-4'

9O

.E

=.. -¢: I/I

u

.5"

._=

-6"

o J~ I--

-d

'

-7'

•

I w~Wu~mumo

0

2 w ~ mu~nu=m

•

3 w~4 b,~stmu~te

-8'

0 -9

,

t000

1200

,

1

1400

1600

Temperature

(C)

modifiers. 6'9 Fluorine is very powerful at lowering viscosity, t° Hence, the viscosity of the liquid phase in the ceramic bodies is expected to be lowered at elevated temperature by the addition of bastnasite. Wet chemical analyses of the mullite obtained by leaching the fired bodies indicated that the mullite is characterized by the Al_,O3-rich composition as shown in Table 6. 3.2 Densification of fired bodies

Figure 3 shows dilatation curves for the mullitecomposition green compacts with and without addition of 3% wt bastnasite. As can be seen, the valley is at 1375cC for the compact with 3% wt bastnasite added and at 1425°C for the compact without bastnasite added. These correlate very well with the second peaks at 1368'~C and 1412°C, respectively, in DTA (Fig. 1), which are related to the mullite formation temperature. Figure 3 shows that the peak for the compact with

(rain)

bastnasite added is 1510~C and that for the compact without bastnasite addition is 1580°C. At 1580°C the linear shrinkage of the former is - 8 % and that of the latter is - 6 % . The bastnasite addition can promote the densification of the fired bodies. Figure 4 shows that at 1550°C densification increases with increasing amount of bastnasite addition and firing time. The compact with 3% wt bastnasite addition can reach approx. 96% theoretical density in 250min, after which the density levels off. Figure 5 shows the same trend at 1600°C. However, the discrepancy between the densification of the compacts with 2 and 3% wt bastnasite added diminishes and densification does not increase much further. It only reaches approx. 96% theoretical density in 175min for the compacts with 2 and 3% wt bastnasite added, whilst the densification of the compacts without bastnasite added continues to increase. In Fig. 6, at 1650°C the densification of the compacts with 0, 1 and 2% wt bastnasite added increases gently with time. The compacts with 1 and 100

~s5o~

16S0~

95 >"

m m r-

p (B

3O0

Fig. 5. Densification of mullite bodics with mullite compositions, by addition of 0-3% wt bastnasite at 1600"C.

100

"o

2OO Time

Fig. 3. Dilatation ofmullite composition green compacts with 0 and 3% wt bastnasite added. Heating rates: below 1000'C, 10~C/min: above 1000"C, 3'C/rain.

>,

100

95

90

_o 90 o

85

[]

k-

80

I

0

,

i

100

=

0 wr% b,t=trBlae

•

t ~

O

2 wt'% ba=ma~le

•

3 ~ ' ~ balu~tldte

buutt.~m

I

I

I

200

300

40O

Time

O o J= I,-

,

ra

85 50O

(rain)

Fig. 4. Densification of ceramic bodies with mullite compositions, by addition of 0--3% wt bastnasite at 1550°C.

' 0

0 ~fl% b r o w

•

t w~ ~truuum

0

2 w?4 =-=,.t~m~to

•

3 wW. k - - m m m

I

I

I

I

30

60

90

120

Time

150

(rain)

Fig. 6. Densification of ceramic bodies with mullite compositions, by addition of 0-3% wt bastnasite at 1650°C.

Mullite.[brmation in presence of bastnasite

• ~--,i, "~. .~.

"

~

35

~

..

I.,C,

-

um

(B)

io ;am

"

".r

-L

~,.

,

%

L~

-.L'h

,oW% Fig. 7. Scanning clectron micrographs of fired ceramic bodies with mullite compositions with the addition ofIAI 0, (B) 1, (C) 2 and (D) 3% wt bastnasite (lower magnification).

2% wt bastnasite added reach approx. 96% theoretical density in 90rain, whilst the compact with 3% wt bastnasite added only reaches approx. 94% theoretical density in 10min, and its densification stops when heating continues. The SEM micrographs (Figs 7 and 8) show that the pores in the fired bodies with bastnasite added are smaller and uniformly distributed, whilst localized, larger pores are found in the fired bodies without bastnasite added; there are no significant morphological changes in the mullite with and without addition of bastnasite, other than grain growth increases as more bastnasite is added• Figure 8 suggests that the essential part of the densification process is the solution and reprecipitation of solids to give increased grain size and density. The slowing down and stopping of the densification process, as shown in Figs 4-6, are caused most probably by the formation of a mullite skeleton. 9

3.3 Slag resistance of the refractories A visual examination on the cross-section of the crucible cut at the center after the slag test was not able to distinguish the difference in the slag resistance of the refractories with various amounts of bastnasite added or without bastnasite addition• Apparently, bastnasite is not an effective additive for enhancing the slag resistance of the refractories for use above 1500°C. A plot of the relative wear rate against the amount of bastnasite added reveals that the slag resistance of the refractories increases slightly as the amount of bastnasite added is raised (Fig. 9). The slight improvement in slag resistance is most probably caused by densification of refractories in the presence of bastnasite. Similar refractories with bastnasite added showed good slag resistance in melting aluminum alloy 7075. 6 At the working temperature (810°C) little

C.-F. Chan, Y.-C. Ko

36

Fig. 8. Scanningelectronmicrographsof firedceramicbodies with mullitecompositionswith the addition of(A) 0, (B) 1,(C) 2 and (D) 3% wt bastnasite (higher magnification).

liquid phase appears in aluminosilicate refractories, even if they contain a considerable amount of fluxing agents. 1~ Moreover, at 810°C the viscosity of the liquid phase in the refractories which were used for melting aluminum alloy 7075 must be very high compared with the viscosity at working temperature above 1500°C for steel-plant refractories. Hence, the refractories have good slag resistance when used for melting aluminum alloy 7075 at 810°C.

3.4 The impurities in bastnasite As can be seen from Table 2, the bastnasite contained approx. 6.9% CaO + BaO + F - . These fluxing agents make a great contribution to producing a large amount of liquid and lowering its viscosity. The impurities in the bastnasite play an important role in enhancing mullite formation,

densification of the fired bodies and slag resistance of the refractories.

4 SUMMARY

AND CONCLUSIONS

Ceramic bodies of mullite composition were prepared from a mixture of calcined alumina and raw kaolin with the addition of bastnasite up to 3% wt and fired at 1400-1650°C. XRD revealed that mullite formation increases with increasing temperature and amount of bastnasite added, but the effect of bastnasite addition diminishes rapidly with increasing temperature. The peak temperature in DTA and the valley temperature in the dilatometric analysis, are indicative of mullite formation; the former was found to be 1412°C without bastnasite addition and 1368°C with 3% wt bastnasite addition whilst the latter was found to be 1420°C without

Mullite formation hi presence o f bastnasite

=,

37

20

A C K N O W L E D G EM ENTS

16

The authors thank J. Y. Huang (General Manager, Steel and Aluminum R & D Department), G. H. Cheng (Assistant Vice President, Technology Division) and J. C. Tsou (Vice President, Technology Division) of China Steel Corporation, for their support.

12

0

t

I

0

5 Bastnaslte

I

/

10

15

content

REFERENCES 20

(wt%)

Fig. 9. Slag resistance of refractories: ©, fired at 1300"C for 3 h prior to slag test at 1500'C for 3 h: Q, fired at 1450"C for 3 h prior to slag test at 150OC for 3 h. Each test was performed for two slag charges.

bagtnasite addition and 1350°C with 3 % w t bastnasite addition. The ceramic green compacts with bastnasite added can only reach approx. 96% theoretical density, at best, when heated at 1550-1650~'C. The pores in the fired bodies with bastnasite added are smaller and uniformly distributed, whilst localized, larger pores are found in the fired bodies without bastnasite added. There are no significant morphological changes in the mullite with and without bastnasite addition, other than grain growth increases as more bastnasite is added. The A1203-rich composition is characteristic of the mullite. The slag resistance of the refractories increases slightly with an increasing amount of bastnasite (0-15% wt) added to the bonding matrix. Bastnasite is not an effective additive for enhancing the slag resistance of steel-plant refractories whose working temperature is above 1500°C.

1. SKOOG, A. J. & MOORE, R. E., Refractory ofthe past for the future: mullite and its use as a bonding phase, Am. Ceram. So('. Bull., 67 (1988) 1180-5. 2. ALPER, A. M. (Ed.), HLgh Temperature Oxides, Part IV, Academic Press, New York, 1971, pp. 61-4. 3. RODRIGO, P. D. D. & BOCH, P., High-purity mullite ceramic by reaction sintering, h~t. J. High-Technol. Ceram., I (1985) 3-30. 4. BAUDIN, C. & MOYA, J. S., Influence of titanium oxide on the sintering and microstructural evolution of mullite, J. Ant. Ceram. Sot., 67 (1984) C 134-6. 5. O'BRIEN, M. H. & AKINC, M., Role ofceria in enhancing the resistance of aluminosilicate refractories to attack of molten aluminum alloy, J. Am. Ceram. Sot., 72 (1989} 896-904. 6. O'BRIEN, M. H. & AK1NC, M., Reduction in aluminum alloy attack on aluminosilicate refractories by addition of rare-earth oxides, J. Am. Ceram. Soc., 73 (1990)491-5. 7. CULL1TY, B. D., Elements of X-Ray Diffraction, 2rid edn, Addison-Wesley, Reading, MA, 1973, pp. 107-419. 8. HALL, J. L., Secondary expansion of high-alumina refractories, J. Am. Ceram. Soc., I 1 (1941)349-56. 9. KINGERY, W. D., BOWEN, H. K. & UHLMANN, D. R., hm'o&u'tion to Ceramics, 2rid edn, John Wiley, New York, 1976, pp. 95 100, 498-501. 10. BODSWORTH, C. & BELL, H., PIo'sical Chemistry o['lron uml Steel Mamtfacture, 2rid edn, Longman, London, 1972, pp. 81-2. I1. N A D A C H O W S K I , F., The assessment of the phase composition of fireclay refractories based on the ternary phase diagrams, R¢:#act. J., 4 (1965) 126-31.