Utilization of Virginia Kyanite and Virginia Mullite in Refractory Applications Worldwide

By, Steven Ashlock 
Director of Technology and Research , Ceramic Engineer , Kyanite Mining Corporation  

ABSTRACT  

The sillimanite group of minerals have long been key components of refractory recipes worldwide. These aluminosilicate minerals exhibit properties that can be utilized in the fabrication of higher end refractories such as high hot strength, excellent creep resistance, and the conversion to mullite at temperature. Of the sillimanite group minerals, kyanite stands out for having the largest expansion during heating at the lowest conversion temperature. This property can be utilized to counteract shrinkage of the refractory mix as well as increasing the density of the mix using fine mesh kyanite. This paper will discuss various uses of Virginia KyaniteTM and its calcined form Virginia MulliteTM in the refractory industry worldwide. 

INTRODUCTION 

Sillimanite Minerals 

Raw materials with a high alumina content are commercially important to the production of refractories. With a medium high percentage of alumina (roughly 60-70%), the sillimanite group of minerals have been a staple of the refractory raw materials industry for many years. The three sillimanite minerals of commercial importance for refractories are kyanite, andalusite, and sillimanite.1 Kyanite, sillimanite, and andalusite are polymorphs with the composition Al2SiO5. Their property differences can be attributed to the unique crystal structures of each of the three minerals. The crystal structure is determined by the temperature and pressure of formation, as shown in Figure 1.2 

 Figure 1: This Pressure-Temperature phase diagram shows the conditions required to form the different minerals in the sillimanite group.

When the sillimanite minerals are heated, they undergo a phase transformation and create mullite and excess silica. (Equation 1) During the conversion, the Al3+ cations in the crystal must rearrange due to a change in coordination number.3 This causes an irreversible expansion as the sillimanite mineral converts to mullite. The amount of expansion during this conversion, as well as the energy required (firing temperature) for full conversion is different for each of the three minerals.4 Kyanite expands the most of the three minerals (17 volume percent) at the lowest required energy input (1400°C firing temperature). These expansion characteristics make kyanite the primary choice for combating shrinkage in a refractory brick or monolithic. Sillimanite expands the second most (6 volume percent) but requires very high firing temperatures (1700°C) to reach full conversion. Most refractories are not fired to such a high temperature, so sillimanite is primarily used as a stable (in terms of expansion) high alumina content filler. Andalusite expands the least of the three minerals at only 4 volume percent. Full conversion of andalusite to mullite takes place around 1600°C. Some companies do use andalusite for expansion, but the expansion is less, requiring more andalusite in the mix, and the firing temperature must be higher, requiring more energy usage. Due to the higher full conversion temperature and large aggregate size availability (not found in either kyanite or sillimanite), andalusite is mainly used as a primary aggregate in a refractory recipe.  

 3(𝐴𝑙2𝑆𝑖𝑂5)−→ℎ𝑒𝑎𝑡3𝐴𝑙2𝑂3∙2𝑆𝑖𝑂2+𝑆𝑖𝑂2

In order to fully take advantage of the properties of the sillimanite minerals, they must be used in as pure a form as possible. Kyanite Mining Corporation is the only supplier of high purity commercial grade kyanite in the world, Virginia KyaniteTM. Virginia KyaniteTM is a highly refined mineral where sophisticated processing and quality control techniques are used in every step of the beneficiation process to ensure a consistent high quality kyanite concentrate.5 This creates a final product that does not contain any of the other sillimanite minerals and has very low impurities, such as an iron oxide level around 0.5%. The lack of impurities creates a raw material with very predictable expansion characteristics that allow it to be used in a variety of refractory applications.  

Expansion of Kyanite 

To understand potential uses, the difference of volumetric expansion and apparent linear expansion must be examined. All kyanite expands 17 volume percent, yet no refractory mix will expand to the full 17 percent. This is due to the particle packing effect. The overall refractory body can only expand if the kyanite is pushing on its nearest neighbor particles. Larger particles of kyanite are more likely to be in physical contact with other particles in the mix and will push on them more than small particles. The smaller particles often lie in between larger aggregates. When they expand, they fill up the void space instead of pushing on the neighboring particles and thus impart less to the overall apparent linear expansion. Table 1 shows the effects on the size of the kyanite particle to the apparent linear expansion of the refractory. In this test, the various sizes of Virginia KyaniteTM were added to a 60% alumina refractory mix at 7%.  

Table 1: The apparent linear expansion of the castable depends
on the size of the kyanite crystals used in the recipe 

Shape of Kyanite 

An important aspect of kyanite is the shape of the material. Kyanite is triclinic, meaning it has three unequal axes and three unequal angles between the vector lines of the crystal structure. For kyanite, this means the crystals have a blade-like shape with a high aspect ratio. This is very different the standard rounded particle shape of many of the materials used in refractories. This elongated shape helps increase strength of the refractory as the long blades act as fibers to aid in crack mitigation. The shape of kyanite also helps with particle packing as the blades can fit in between the other spheroidal particles in the mix, helping to increase density.  

Calcination 

Virginia KyaniteTM can also be calcined into Virginia MulliteTM (shown in Equation 1), creating the only mullite made from kyanite in the world. This material is inherently different than mullite created by calcining clay minerals due to its origin as kyanite. Mullite made from calcining kyanite retains the blade shape of kyanite through the phase transformation, creating a large crystal with a high aspect ratio. This is different than other mullites that are created from extruding or spheredizing clay and crushed and are therefore somewhat round. This does mean that the mullite made from kyanite has size limitations. The particles can only be as large as the starting kyanite crystals so large aggregates cannot be made. The largest size is 20x50 mesh (840x300 µm). The shape of the mullite crystals and the low impurities due to them being removed in the purification of the kyanite create a raw material that exhibits attractive high temperature properties such as excellent resistance to creep.   

APPLICATIONS FOR VIRGINIA KYANITETM IN REFRACTORIES 

Monolithics 

Monolithics are by far the largest user of kyanite around the world. The predictable expansion of kyanite during the firing of the monolithic is utilized in a variety of ways. The coarser sizes, -40m (420 µm) and -50m (300 µm), are commonly used to control the amount of shrinkage in the castable. It is very common to also pair coarse kyanite with the finer mesh sizes, -200m (75 µm) and -325m (45 µm). The addition of the fine mesh material increases the density of the refractory by filling in the void spaces upon expansion. The phase transformation of the fines also provides a reliable source of mullite in the matrix which aids in thermal shock resistance. The increase in mullite in the matrix as well as the decreased porosity help to increase the service life of the refractory. The size and amount used are dependent on the amount of expansion desired and the particle packing on the mix.  

In the case of low cement castables and pre-cast shapes, -50m (300 µm) and -100m (150 µm) are the most used material. Additions of around 4-8% are commonly used to offset shrinkage. The -100m material contains more small particles than the -50m material and introduces some fines to aid in densification of the castable. However, more of this material needs to be used to offset shrinkage as the material imparts a smaller apparent linear expansion to the refractory. Many applications work better with a mixture of the -50m (300 µm) for shrinkage control with additions of -325m (45 µm) for the increased density.  

High cement conventional castables typically require the use of the coarsest size material, -40m (420 µm) to offset the large shrinkage. Amounts typically range from 10-15% but vary widely based on the amount and quality of the cement used as well as the service temperature. 

Plastic refractories typically use both -40m (420 µm) and -325m (45 µm) kyanite. The coarse -40m (420 µm) product is used to offset the large amount of shrinkage prevalent in these types of castables. Plastic refractories utilize the -325m (45 µm) kyanite to introduce fine mesh mullite to the matrix which helps to improve the hot properties. The amount of kyanite varies, but 8-12% of the -40m (420 µm) and 4-6% -325 (45 µm) is typical. 

Most ramming and gunning mixes use either -50m (300 µm) or -100m (150 µm). The amount used is based off the expected performance of the refractory as well as the other constituents of the mix.  

Dry and damp vibrateables can use a variety of sizes based on the particle packing of the other ingredients in the mix. The most common sized material is the -100m (150 µm) as it has large particles to combat shrinkage but also contains fines to broaden the particle size distribution of the refractory mix, making it easier to vibrate into place. The -325m (45 µm) is used to improve density by closing off porosity. The latter is typically used at around 3-5% while the former is used at 5-10%. 

Mortars use the highest amount of kyanite at 20-30%, based on the service temperature of the mortar. The common sizes for this application are the -50m (300 µm), -200m (75 µm), and -325m (45 µm). Large amounts of the -50m (300 µm) are used to offset the large volume shrinkage while the finer mesh materials improve the hot properties of the matrix. 

Tap hole clay is important application where kyanite is commonly used. Recipes of tap hole clay vary widely, and no two mixes are the same. Tap hole clay is an application that needs a lot of expansion to create the mushroom inside the blast furnace. The high expansion percentage of the -40m (420 µm) kyanite is an important ingredient in tap hole clay. 4-6% is very common. Other sizes can also be used based on the particle packing of the mix. Finer mesh sizes help to provide a source of mullite to the matrix, adding hot strength to the mushroom. 4-6% of the -325m (45 µm) is common for this reason. The mullite created during the phase transformation from kyanite is friable, making the tap hole clay easier to drill out during the next tapping. 

Bricks  

Kyanite is commonly used in a variety of bricks. The primary reason is to control overall shrinkage of the bricks. The reduction of porosity/increasing density by allowing the fine mesh kyanite to expand into the void spaces is also a common application in bricks. Not only does the expansion of fines close off porosity, it also creates mullite in the matrix of the brick after pre-firing. This mullite is highly creep resistant due to the low amount of impurities and aids in the thermal shock resistance of the brick. The largest user of kyanite in the brick industry are carbon baking furnace bricks for the aluminum industry. Kyanite additions are also very common in insulating fire bricks. The amount of kyanite varies greatly for these bricks depending on the desired final product characteristics, but 10-20% is typical. In the case of insulating fire bricks, the finer mesh kyanite is not typically used as porosity is encouraged for insulation. High alumina bricks are another industry where kyanite is used for shrinkage control, increased density, and thermal shock resistance. 

Kiln Furniture 

Kyanite additions to mullite/cordierite kiln furniture are very common to control shrinkage as well as to provide another source of mullite after firing. Most mullite/cordierite kiln furniture is pre-fired to below the full conversion temperature of kyanite, meaning that only some of the kyanite will be converted to mullite on the initial firing cycle. When put in use, the kiln furniture will see repeated heating and cooling and cause the remaining kyanite to convert to mullite over time. This imparts volume stability as the glassy phases in the kiln furniture begin to soften and creep. The most common size for this application in -100m (150 µm). 

Foundry 

Kyanite can also be used as a slurry in the foundry industry for several casting methods. Centrifugal and lost foam are the most predominate casting methods that use kyanite. For this application, -200 (75 µm) or -325 (45 µm) are used as a spray on the inside of the sand mold or outside of the foam mold. As the metal is poured, the kyanite expands instantaneously (if the metal pouring temperature is above the kyanite transition temperature) when it comes in to contact with the metal and allows the gases to escape. This reduces the number of defects in the metal. The friable ceramic coating also makes it easer to remove the casting from the mold after cooling.  

Replacing Silica Fume 

A recent development has been the use of large amounts of kyanite in refractories for the aluminum industry. Extensive post-mortem research on refractories for aluminum has shown that kyanite crystals are un-touched by molten aluminum metal while most of the other materials were attacked and underwent severe degradation.6 Since the initial attack was observed on the silica fume, research was done to try and replace it in the mix. It was discovered that very fine mesh kyanite (d50=5-10 µm) could partially replace the silica fume and improve corrosion resistance. It needs to be a partial replacement as the silica fume is still needed to aid in flow of the castable. This replacement has been utilized in a variety of applications, not just aluminum, as silica fume is often the first place of attack in a refractory. 

Use of Kyanite in Aluminum Refractories 

Understanding kyanite to be resistant to aluminum6, coarser sizes have been used in refractories for aluminum in amounts up to 40%. Typically, kyanite cannot be used in such a large quantity in a mix as it will impart a considerable amount of expansion that would be detrimental to the refractory. However, aluminum metal applications rarely exceed 1250°C, which is well below the conversion temperature of the kyanite, so there is no danger of excessive volumetric expansion. Using a large amount of kyanite fines helps to enhance the corrosion resistance of the matrix and protects the larger aggregates in the mix.  

Partial Replacement of Calcined Alumina 

In the past several years, refractory companies around the world have been successful in partially replacing calcined alumina with kyanite in refractory mixes. Research has shown that replacing part of the calcined alumina with fine mesh kyanite (-325m or 45 µm) helps to improve physical and high temperature properties despite lowering the overall alumina content.7 The fine mesh kyanite does two key things in the mix. First, the kyanite expands to fill the void spaces during the phase transformation, filling in the porosity of the refractory and increasing density. Secondly, there is a small amount of silica created during this conversion (see Equation 1). This silica is very highly reactive and will readily react to form secondary mullite with any available alumina. Therefore, this application only calls for a partial replacement of calcined alumina. The remaining calcined alumina reacts with this silica and forms a fine mesh mullite in the matrix, increasing the hot properties of the refractory. While it is possible to form this secondary mullite by combining the calcined alumina with the silica fume, it has been found that this reaction occurs at significantly higher temperatures than the kyanite conversion temperature, requiring more energy input.1,7,8 

APPLICATIONS FOR VIRGINIA MULLITETM IN REFRACTORIES 

Bricks and Monolithics 

Mullite is used in refractories of all types where good thermal shock resistance, high hot strength, and high creep resistance are required. Virginia MulliteTM is used for its superior resistance to creep in the 60% alumina range of materials. It is primarily used in its finer mesh forms for low cement castables, mortars, bricks, and dry vibrateables.  The very low amount of impurities makes it an excellent choice in applications where high creep resistance is crucial to the success of the refractory such as side-wall bricks.  

Kiln Furniture 

Virginia MulliteTM is also widely used in kiln furniture due to its excellent thermal shock resistance. The low level of impurities in the material provides excellent creep resistance to the kiln furniture, increasing the number of cycles before replacement. The sizes most used for this application are -40m (420 µm) and -325m (45 µm).  

Foundry 

Foundry washes and sprays are a growing market for Virginia MulliteTM. The mullite is very thermally stable and acts as a refractory layer between the mold and the molten steel. The low level of impurities creates a material that undergoes very little mold-metal reaction and has even been able to replace materials like zircon in applications that do not exceed 1600°C. The lack of mold metal interaction creates a casting with a very smooth surface finish and makes it easy to remove the casting from the mold after metal solidification. 

CONCLUSION 

The beneficiation of the kyanite ore body by Kyanite Mining Corporation creates a commercially available product that is very low in impurities. This high-quality material can be used in raw or calcined form in a wide variety of applications in the refractory industry. Of the minerals in the sillimanite mineral family, kyanite expands the most at the lowest firing temperature, making Virginia KyaniteTM the material of choice for shrinkage control in refractories. The expansion of kyanite to mullite during the phase transition can also be utilized to reduce porosity and increase density by adding fine mesh kyanite to the refractory recipe. The mullite made from Virginia KyaniteTM exhibits excellent hot properties such as creep resistance and can be utilized in many demanding applications. 

REFERENCES 

1 Brown, Jesse. Kyanite. Report. Materials Engineering, Virginia Polytechnic Institute and State University. 1982. 

2 Joensson, B. and Sundman, B. “Thermochemical Applications of Thermo-Calc.” High Temp. Sci., vol 26, 1990. pp. 263-273. 

3 Brandt, Richard C. "The Sillimanite Minerals: Andalusite, Kyanite, and Sillimanite." Ceramic and Glass Materials Structure, Properties and Processing. By James F. Shackelford and Robert H. Doremus.: Springer, 2008. pp. 41-8.  

4 Ashlock, Steven. “A Property Comparison of Commercially Available Sillimanite Minerals,” EUROGRESS, 1-10 (2017). 

5 Ashlock, Steven. “How it’s Made: Premium Grade Virginia Mullite,” Incast, vol 33, no. 2, 2020, pp. 20-26. 

6 Headrick, William L. et al. “Qualification of Micronized Kyanite and Silica Fume.” Materials Science and Technology, 15-18 Oct. 2006. 

7 Ashlock, Steven and Dilip Jain. “Examining the Effects of the Partial Replacement of Calcined Alumina with Kyanite in a 60% Low Cement Castable,” ALAFAR, 1 Oct. 2018.  

8 Jain, Dilip. “Mullite Formation: A Myth of Reality?.” St. Louis Section of the American Ceramic Society, 29 Mar. 2007.