Examining the Effect of the Partial Replacement of Calcined Alumina with Kyanite in a 60% Low Cement Castable
Steven Ashlock
Kyanite Mining Corporation
Dillwyn, VA, USA
Dilip Jain
Kyanite Mining Corporation
Dillwyn, VA, USA
ABSTRACT
The partial replacement of calcined alumina with Virginia KyaniteTM in a 60% LCC in an attempt
to enrich the matrix with mullite was examined. Previous work has shown that calcined alumina
does not react as readily as believed to form secondary mullite. Bars of three different castable
mixes were prepared and tested for MOR, CCS, density, and other properties. Chemistry and
phase composition were evaluated using XRF and XRD respectively. Despite the lowering of the
overall alumina content the majority of the castable properties were improved. The addition of -
325m kyanite created mullite in the matrix after sintering to 1480°C and higher. In all three mixes
some calcined alumina remained unreacted even after firing to 1600°C. The density is also
increased as the -325m kyanite expands into and reduces the porosity of the refractory.
INTRODUCTION
The demands on refractories in the modern market are constantly changing and becoming more
challenging.1 Requests for longevity increases are commonplace from the buyer to the refractory
supplier. In order to achieve this goal companies are increasingly enhancing the overall alumina
content. This requires using more expensive higher end raw materials. One such material that is
a staple of many high end alumina refractories is calcined alumina. Calcined alumina is added to
improve refractoriness to the matrix.2,3 It is also said to react with silica fume to form interstitial
mullite.4,5 It has long been known that mullite in the matrix greatly enhances the thermal shock
capabilities of the refractory. However, research shows that the reaction between silica fume
and/or silica and calcined alumina takes place at fairly high temperatures.6,7 Most refractories
would need to be heated to well above usage temperature in order to see this beneficial reaction.
Virginia KyaniteTM is a member of the sillimanite group of minerals.8 The minerals that make up
this group are kyanite, sillimanite, and andalusite. These minerals are used in a wide variety of
refractory applications. One unique property of the sillimanite group minerals is conversion to
mullite after calcination via the following reaction:
Kyanite has the lowest temperature of conversion of the group (1430°C) as well as the largest
amount of expansion upon conversion at 17 volume percent. This expansion is used throughout
the industry as a means of offsetting shrinkage of the other minerals in the refractory recipe. To
achieve this goal the larger commercially available sizes of kyanite, such as -35, -48, and -100
mesh (425, 300, and 150 μm), are used. The finer mesh sizes such as -200m (75 μm) and -325m
(45 μm) are used to increase the density of the refractory. Most of the expansion of this finer mesh
kyanite is absorbed by the porosity thus increasing the bulk density. Fine mesh kyanite has
another attribute than can be exploited: mullite formation in the matrix of the refractory.
TEST PROCEDURE
Three 60% alumina low cement castables (LCCs) were created to observe the change in castable
properties as calcined alumina (A35-325) is replaced with Virginia Kyanite. The formulations,
based off of mullite aggregates, are listed in Table 1. As the amount of 325m kyanite is increased
the amount of 48m kyanite needs to be decreased. Too much kyanite in the refractory mix will
cause an overexpansion that is detrimental. After mixing and water additions the mixes were
vibration cast in 25x25x152 mm molds. The bars were allowed to cure overnight before
demolding. The bars were then dried at 110°C before firing in an electric furnace. Firing conditions
were a ramp of 5°C per minute with five hour hold times at either 1370°C, 1480°C, or 1600°C with
a natural cool down.
Tab. 1: LCC mix recipes
Various castable properties were measured for each series of bars. Permanent linear change
(PLC) was measured using calipers before and after firing. Bulk density, apparent porosity, and
water absorption were measured using ASTM C20-00 (2015). ASTM C133-97 (2015) was used
to test modulus of rupture (MOR) via three point bend and cold crushing strength (CCS) on
25x25x25 mm cubes. Chemical analysis was performed via X-Ray fluorescence (XRF) on a
Panalytical PW2400 as well as phase analysis by X-ray diffraction (XRD) on a Panalytical Cubix3
utilizing the Rietveld method.
RESULTS AND DISCUSSION
Chemistry
The chemistry results obtained via XRF are shown in Table 2. The most noticeable chemistry
change is the decrease in alumina with increasing amount of -325m kyanite. This increase in
kyanite, as a replacement for the pure calcined alumina, also slightly increases the iron and titania.
Tab. 2: Chemistry data of the castables in oxide percentages
Physical Properties
Bars of all three recipes were then fired and had their physical properties tested. The bulk density
data is shown in Figure 2. The bulk density of all three castables was the same after firing to
1370°C. As the firing temperature increases the differences become apparent. Mix 3, the mix with
the most -325 mesh kyanite, has the highest bulk density of the three mixes at both 1480°C and
1600°C. This is due to the high amount of expansion into the porosity of the refractory. This mix
is also the only mix to maintain or even increase it’s density with a higher firing temperature. Mix
1 and Mix 2 both had a lower density after firing to 1480°C but regained some of this value when
firing to 1600°C. This decrease at 1480°C is likely due to the expansion of the 48m kyanite. Mix
1 contained more 48m kyanite thus the larger decrease in density. Evidence of this expansion
into the pores was verified by looking at the apparent porosity. As the -325 kyanite increased the
apparent porosity was lowered.
Fig. 2: Bulk Density
The permanent linear change (PLC) of the bars was examined. A graphical display of this data is
shown in Figure 3. At 1370°C little to no expansion is seen. This is to be expected as this is below
the temperature of expansion for kyanite. At 1480°C the expansion becomes more apparent due
to the phase transformation and expansion of kyanite. All three mixes experienced further
expansion when fired to 1600°C as amorphous is formed and bloating occurs. As seen in the
XRD data examined later in this paper, a small additional amount of mullite is also formed after
the 1600° firing. The secondary mullite formation contributes to the expansion at 1600°C. The
mixes with higher amounts of 48m kyanite (Mix 1 and 2) showed the largest expansion at both of
the higher temperatures.
Fig. 3: Permanent Linear Change
The cold crushing strength (CCS) of the mixes was then tested (Figure 4). The mixes containing
-325m kyanite showed greater strength than the mix without at both of the lower temperatures.
At 1600°C Mix 1 (89.5 MPa) actually had an almost equal CCS value to Mix 3 (89.4). Mix 3 had
a lower CCS value than Mix 2 at every temperature. Mix 3 has the lowest alumina percentage
and likely created the most amorphous phase when fired thus weakening of the mix. Mix 2 had
the highest CCS values at all three temperatures. Previous studies on this system of mixes
showed increasing strength with increasing 325m kyanite meaning Mix 3 was expected to exhibit
the highest CCS values.7 Results in this test were contradictory to our previous work and another
study needs to be done to clarify the discrepancy.
Fig. 4: Cold Crushing Strength
The modulus of rupture (MOR) was the last of the physical properties tested. Results (Figure 5)
showed higher results with an increase in the -325m kyanite. This trend is seen at all three firing
temperatures with the exception of Mix 3 at 1600°C in comparison to mix 2. Gains in strength,
while minimal, were seen as the mixes were fired to higher temperatures. The lower MOR values
of Mix 3 at 1600°C are likely a result of the mix having the lowest overall alumina content and
thus the highest amount of amorphous material present.
Fig. 5: Modulus of Rupture (3-point bend)
Phase Analysis
Each of the three mixes was examined using XRD for the presence of kyanite and corundum at
all three temperatures. The goal was to determine that a) the kyanite had converted to mullite and
b) to determine if there was any residual corundum or if it had been converted to mullite as well.
To examine our first goal we look at Figure 6. This shows Mix 2 after firing to the three different
temperatures. Here we can see the kyanite being converted at 1480°C. The kyanite peak at 28.0
has been highlighted for ease of viewing. Similar trends are shown in all three mixes.
We also can see from the XRD values that there is still some residual corundum in all three of the
mixes even after firing to 1600°C. This can also be seen when looking at Figure 7. The corundum
peak at 43.34 has been enlarged to show it is still present in the sample. The scan showing the
highest percentage of corundum is Mix 1 with the least amount being shown in Mix 3.
Fig. 6: The kyanite peak is present at 1370°C but is gone upon conversion after 1480°C.
Fig. 7: XRD shows that corundum is still present even after firing to 1600°C.
All three of the mixes showed an increase in mullite as the temperature increased. This can be
seen in the scans peak height and in Rietveld analysis. The largest gain in mullite percentage
was seen between 1370 and 1480°C. This is to be expected as the kyanite is converting to mullite
between these temperatures. A smaller gain is seen between 1480 and 1600°C. This is a clear
indication that some secondary mullite is being formed in the firing process. However, as stated
earlier, there is still corundum present even at 1600°C. Unfortunately, the secondary mullite
formation did not completely consume all of the calcined alumina intended for that purpose. Mix
3 had the highest amount of mullite present after each of the higher firing temperatures. This is to
be expected as Mix 3 contained the highest percentage of kyanite and thus the highest amount
of mullite formation not associated with secondary mullite formation involving calcined alumina.
CONCLUSION
The partial replacement of calcined alumina with -325 mesh kyanite showed several advantages.
Density increases were seen as -325m kyanite was introduced and allowed to expand to fill the
porosity of the castable. The addition of -325 mesh kyanite also improved both MOR and CCS
when the amount of calcined alumina and- 325 mesh kyanite were equal. At a higher percentage
of kyanite these values lowered, likely due to the increasing amount of impurities in the castable.
Further testing would need to be done in order to determine the ratio of calcined alumina to kyanite
at which the properties begin to decline. Although the overall alumina content is reduced with
kyanite addition, the replacement of calcined alumina with -325 mesh kyanite appears to be
beneficial. Lastly, despite popular belief, XRD analysis showed that even when firing to 1600°C
calcined alumina did not completely react with silica to form secondary mullite. Replacing some
of the calcined alumina with -325m kyanite guarantees the formation of mullite in the matrix.
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