Application Field: Secondary Mullite Formation for Refractories and Ceramics
In our last two posts (found here), we explored how kyanite is used for shrinkage control and reducing porosity in refractories and ceramics. Today, we’re throwing another application into “the mix”.... Let’s talk about secondary mullite formation.
What do you mean by Secondary Mullite?
Mullite can be incorporated into a refractory mix in two ways: by adding mullite aggregates directly or by converting other raw materials into mullite during heating (in-situ). The latter is known in the industry as secondary mullite formation. In our case, this is done by converting kyanite into mullite. Alternatively, secondary mullite can be created by reacting silica and alumina at high temperatures.
Why Is Secondary Mullite Formation Important?
Refractories and ceramics are constantly subjected to chemical attacks, physical erosion, and abrasion. Chemical attack commonly begins in the refractory matrix, the mixture of finer sized materials around the larger aggregates. Often, the silica fume present in the matrix is the first material attacked.
To combat this, many companies mix the material with fine mesh alumina to create secondary mullite, which offers superior resistance and durability. The challenge? Even with the high temperatures required for this reaction, the alumina doesn’t always fully react with silica to achieve secondary mullite formation.
This is where kyanite comes in…
Using Kyanite for Secondary Mullite Formation
When heated, fine mesh kyanite (200m or 325m) transforms into mullite, providing the benefits of the mullite phase to the matrix. This conversion happens at a much lower temperature than the silica/alumina reaction, meaning that you get mullite in the matrix without the need for as much heat (energy), saving on firing costs.
The phase transformation from kyanite to mullite also creates incredibly fine (and reactive) silica. This silica readily reacts with the alumina found in the matrix to make even more secondary mullite. Studies have shown that mullite made from kyanite also acts as a seed crystal, helping to drive further reaction of the remaining alumina with the silica fume.
The result? Longer-lasting refractories, less maintenance, and improved operational efficiency—win, win, win!
Ready to Optimize Your Mix?
We can help you optimize your refractory mix for maximum performance and longevity. Explore our research on secondary mullite formation here or connect with us directly to get started: sales@kyanite.com | +1 (434) 983-2043
