Inside Mullite / Part 2: How It’s Made
Picking up where we left off in Part 1…
Despite its industrial importance, mullite is rarely found in nature. It typically forms under specific high-temperature, low-pressure conditions that are rare in most geological settings. So, for industrial use, mullite must be made—but how?
Let’s break it down.
How Mullite is Made
There are three primary methods for producing synthetic mullite:
1. In-Situ Mullite Formation
In this method, mullite forms directly within a ceramic body during firing. Used as far back as 1500 BC in Chinese porcelain, this remains common in traditional ceramics today. As clay is fired, mullite crystals grow within the structure.
2. Calcined/Sintered Mullite Aggregates
More common in industrial settings, this process involves calcining minerals—typically clay (but also kyanite!)—at high temperatures to form mullite. The resulting material is crushed into aggregates or milled into fine powders (flours), commonly used in investment casting and refractories.
3. Fused Mullite
The third method involves melting pure alumina and silica together to produce fused mullite. This high-purity material (typically 72–74% alumina) is free of impurities and can be formed into various sizes. This method requires a lot of energy, which is reflected in higher costs.
How Virginia Mullite® is Made
While most synthetic mullite is made from clays, Virginia Mullite® is produced by calcining Virginia Kyanite®, a member of the sillimanite mineral family (which includes kyanite, andalusite, and sillimanite).
Here’s a look at the process:
1. Beneficiation
Mined kyanite ore goes through a 25+ step beneficiation process to remove impurities, resulting in 35 Mesh Virginia Kyanite.
2. Calcination
The refined kyanite is heated to over 1450°C, triggering a phase change to mullite. This conversion causes a 17% volume expansion and produces mullite crystals. The resulting mullite retains the original needle-like shape of the kyanite. Chemistry is unchanged during this conversion.
3. Finishing: Screening or Milling
The final product of the calcination process is our 35 Mesh Virginia Mullite. This material can be sold as 35 Mesh or undergo further processing to make various sizes:
Screening: Material is screened to desired sizes (20x50, 50x100 mesh) to create tight particle size distributions for various applications such as stuccos for investment casting.
Milling: Remaining material is milled in ball mills and air-classified for precise particle sizing. This is how we make the 48, 100, 200, and 325 mesh size products. What’s in Virginia Mullite®?
Virginia Mullite® contains ~80% mullite, 11% finely dispersed amorphous silica, 7% quartz, and trace amounts of cristobalite. The alumina content of our Virginia Mullite® averages around 57%.
Selecting a mullite with the appropriate alumina percentage is critical to achieve the desired properties for many applications. For reference, mullite products typically fall into three categories based on alumina content: 45% alumina, 60% alumina, and 70% alumina. Virginia Mullite typically competes in the 60% alumina market.
However, Virginia Mullite can also compete with materials with even higher alumina contents (70%) due to its low level of impurities. Although higher alumina generally means higher temperature resistance, impurities often play an even greater role in performance.
That’s where Virginia Mullite® excels—it’s extremely low impurity levels create a mullite with superior resistance to creep and chemical attack. Our Premium Grade Virginia Mullite, with an even lower iron content of around 0.15%, has been gaining momentum in industry replacing 70% alumina mullites in recent years.
Why Choose Virginia Mullite®?
Virginia Mullite® is mined, processed, and calcined in the U.S. by Kyanite Mining Corporation—delivering a consistent, cost-effective, and readily available product.
Unlike Virginia Kyanite, Virginia Mullite is volume stable at high temperatures and can be used as a larger percentage of your mix. It’s high purity makes it an excellent material for improving thermal shock resistance, chemical attack resistance, creep resistance, and hot strength of any refractory or ceramic application.
Ready to enhance your refractory performance?
Get in touch to see how we can help!
sales@kyanite.com | +1 (434) 983-2043
Coming Soon: Inside Mullite / Part 3: How It’s Used
Stay tuned as we explore how Virginia Mullite® is powering high-performance industries worldwide.
