Mullite Formation A myth or Reality?

Transcription

1 Mullite Formation A myth or Reality? Dilip Jain Virginia, U.S.A St Louis Section Meeting of the American Ceramic Society March 29, 2007

2 It is very well known that when ever there is a source of free silica and alumina in any refractory, Mullite is formed upon heating at high temperatures. Generally it is believed that this can happen at as low a temperature as 1200C. It is not certain however if this actually happens in any appreciable amounts. All monolithics have source of either reactive or calcined alumina, silica fume and other sources of silica and alumina in the matrix.

3 Monolithics are very complex with many sources of Corundum and Mullite and hence it is very difficult to know when and how much mullite is being formed at any given temperature. A controlled study was undertaken to see when and how much mullite is actually formed. Several sources of alumina, silica, mullite and Kyanite were used to make mixtures that has an alumina content of about 70%. These were then heated to a series of temperatures starting from 1000 c and up to 1650 c in 100c increments.

4 After mixing with water and using 2% Norlig A binder, bars 6x1x1/4 inch were pressed then dried and fired in a lab electric furnace at various temperatures. The rate of rise of temperature was 10c per minute and were held at the temperature for one hour. After the heat treatment both XRF and XRD analysis was conducted to see when and how much mullite was being formed..

5 For the XRD a slow scan of degrees was performed and then all the phases were identified using Xpert High Score Plus software. After identifying all the peaks, the peak height of the Mullite peak at degrees (60) and Corundum peak at degrees (48) were used to measure the peak height. These two peaks were chosen as no interference from any other mineral at these peaks were found. Peaks with most intensity for both had interference with other phases.

6 The peak height was measured using the software and the background was subtracted from the peak height and then the ratio of the Mullite to Corundum peak was calculated. This was done to more accurately determine the amount of Mullite and corundum present rather than the absolute intensities of each. Table I lists the various mix designs, Table II lists the chemical analysis of various mixes

7 , It is interesting to note that for pure alumina and silica no mullite formation was seen till a temperature of 1500c. Also even after 1650 c heat treatment, some corundum was present that still had not reacted with silica to form Mullite. It is understood that presence of impurities will lower the formation of Mullite some what but not to an appreciable amount.

8 Mix Table I A C90 LSB /Colloidal Silica 1140 B C90 LSB / Silica Fume 971 C A / Silica Fume 971 D A / Colloidal Silica 1140 E F G HA Micronized Kyanite / C90 LSB Micronized Kyanite / A Alumina Va Mullite 325M / C90 LSB Mulcoa m / C90 LSB K Va Kyanite 325m

9 Table II Mix Al2O3 SiO2 Fe2O3 TiO2 Alkalies A B C D E F G HA K

10 Mullite Peak Mullite Peak A5 (1400C) A7 (1600C) B6 (1500C) B8 (1650C) C6 (1500C) C8 (1650C) D5( 1400C) D7 (1600C) E6 (1500C) E8 (1650C) F5 (1400C) F7 (1600C) G6 (1500C) G8 (1650C) HA5 (1400C) HA7 (1600C)

11 Corundum Peak B6 (1500C) C8 (1650C) D6 ( 1500C) E7 (1600C) F5 (1400C) F8 (1650C) Corundum Peak HA7 (1600C) G6 (1500C) A5 (1400C) A8 (1650C)

12 M/C Ratio M/C Ratio A5 (1400C) A7 (1600C) B6 (1500C) B8 (1650C) C6 (1500C) C8 (1650C) D5( 1400C) D7 (1600C) E6 (1500C) E8 (1650C) F5 (1400C) F7 (1600C) G6 (1500C) G8 (1650C) HA5 (1400C) HA7 (1600C)

13

14

15 , In series A that is reactive Alumina with Colloidal Silica, there is a very small amount of Mullite formation at 1500C. At 1650C there still is residual Corundum left that has not reacted with silica to form Mullite.

16

17

18 , In series B that is a mixture of reactive Alumina and silica fume, there is small amount of Mullite formation at 1500 C. Even at 1650C there still is residual Corundum left that has not combined with silica to form Mullite.

19

20

21 , In series C that is a mixture of calcined alumina and silica fume Mullite formation starts after 1500 C. At 1650 C there still is residual Corundum that has not reacted with silica.

22

23

24 , In series D that is a mixture of calcined alumina and colloidal silica, small amount of Mullite is formed at 1500C. At 1650 C there still is residual Corundum that has not reacted with silica to form Mullite.

25

26

27 , In series E that is a mixture of Micronized Kyanite and reactive Alumina, fair amount of Mullite formation has taken place even at 1400C. This mixture shows the maximum amount of Mullite formed. But at 1650 C still a very small amount of residual Corundum is left that has not reacted with silica to form Mullite.

28

29

30 , In series F that is a mixture of Micronized Kyanite and calcined Alumina, fair amount of Mullite is seen at 1400C. But even at 1650c some residual Corundum is left that has not reacted with silica to form Mullite.

31

32

33 , In series G that is a mixture of VA Mullite 325m and reactive Alumina more Mullite is formed than Mulcoa 60. This is because the silica that is released during conversion is very finely distributed and is amorphous in nature and quite reactive. But even at 1650C some residual Corundum is present that has not reacted with silica to form Mullite.

34

35

36 , In series HA that is a mixture of Mulcoa m and reactive Alumina, less Mullite is formed than Va Mullite as all the silica is in a glass state and not quite reactive. Even at 1650 C residual Corundum is left that has not reacted with silica to form Mullite.

37

38

39 , In series K that is Kyanite 325m, Mullite formation starts at about 1250 C and is complete at about 1450C. It is the lowest temperature than either Andalusite or Sillimanite.

40

41

42 , In this comparison, all of the series at 1500 are compared. From this one can see the relative amounts of Mullite and Corundum are present in each.

43

44

45 , In this comparison all of the series at 1600 are shown. This shows the relative amount of Mullite and Corundum in each.

46

47

48 , In this comparison all of the series at 1650 are shown. One can see the relative amounts of Mullite and Corundum in each.

49

50

51 , In this comparison all of the series with Kyanite and Mullite at 1400 are shown. One can see the relative amounts of Mullite and Corundum in each.

52 , In order to see what happens to the Mullite formation in the presence of Cement, mixture was made with 30% cement, and balance with either alumina and silica or Micronized kyanite and Alumina was prepared and fired to 1400C.

53 Table III Kyanite Secar Reactive Silica % 71% Alumina % Fume % KAS SAS

54

55

56

57 It is interesting to note that there was hardly any Mullite present when a mixture if Secar 71 cement, silica fume and reactive alumina was fired to 1400 c. It shows only Anorthite and Corundum. Where as a mixture of micronized Kyanite, Secar 71 cement and reactive alumina shows Mullite, Anorthite and Corundum.

58 Conclusion 1) Alumina and Silica do not begin to form Mullite at a temperature lower than 1500c. Certainly presence of impurities may change this some what but not dramatically.

59 Conclusion (cont.) 2) As to be expected Colloidal silica combines with both calcined alumina and reactive alumina to form Mullite at slightly lower temperature and in larger amounts than silica fume. Also as to be expected, Colloidal Silica and reactive alumina formed more Mullite than the other two combinations.

60 Conclusion (cont.) 3) Free Silica in Virginia Kyanite in presence of Alumina coverts to Mullite readily and at a much lower temperature. This is because the excess silica that is ejected during conversion of Kyanite is amorphous in nature and is quite reactive similar to Andalucite. This combination produced by far the most Mullite of the series.

61 Conclusion (cont.) 4) Virginia Mullite combines with Alumina to form Mullite more readily than Mulcoa 60. This is because Virginia Mullite does not have any glass phase. All excess silica is in the form of Quartz, Cristobalite and amorphous silica that is finely distributed and is more reactive than Silica in glass.

62 Conclusion (cont.) 5) It is clear that presence of finer Kyanite and Alumina in the matrix will produce Mullite at lower temperatures and in larger amount than other forms of Alumina and Silica that is beneficial.

63 Conclusion (cont.) 6) A separate study has shown that part of the silica fume and alumina can be replaced with micronized Kyanite that will enhance Mullite formation at lower temperature and provide better hot properties.

64 Conclusion (cont.) 7) Virginia Kyanite converts to Mullite in a very narrow range of 1250 c to 1450 c, a temperature much lower than either Andalucite or Sillimanite.