Possible development direction of refractory raw materials

In order to prepare long-life refractory materials, high-quality raw materials are required. The so-called "high-quality" raw materials have been understood in recent years as "purer chemical composition and phase composition" raw materials. This is related to the performance of some of the refractory materials we currently use, that is, the materials react with some components in the corrosive medium (melt, gas phase medium, dust, steam, etc.) to form compounds with high melting temperature and volume stability. When this compound is formed, the lattice in the refractory crystal absorbs a certain number of cations in its own space without significantly changing the lattice parameters. In this valence, the life limit of the refractory material is the limit of the lattice being saturated with the absorbed cations until the lattice parameters begin to change (the structure of the material begins to loosen, melt and weaken).

In the past 30 years, the life of refractory materials has been greatly improved in a simple way: by increasing the purity of the raw materials used, the absorption capacity of the refractory materials is increased, thereby extending their service life. In addition, by improving the perfection of the material structure (using molten materials or high-temperature calcined materials), it is possible to extend the use time before damage. This slows down the dissolution of the aggressive components in the refractory material. Materials that can reduce the wetting of the aggressive components on the refractory matrix (such as large flake graphite, chromium oxide, etc.) are a protective barrier during the penetration of cations in the aggressive medium into the refractory structure. This method can increase the life of the refractory several times. However, this method cannot increase the life of the refractory material indefinitely, because it is impossible to use 100% of the main components in the refractory composition. Moreover, the use of 100% pure materials is complex and expensive.

Looking around at similar material sciences, we can see that similar tasks also exist in similar material sciences, that is, in order to increase the use temperature of materials, it is necessary to solve the problem of creating high entropy materials. High entropy materials are characterized by the disordered distribution of the internal atoms of the cations, which leads to the formation of complex crystal structures and significant lattice distortions. In this case, high entropy can ensure the thermodynamic stability of this type of material at high temperatures.

For example, (Sm0.2Eu0.2Tb0.2Dy0.2Lu0.2)2Zr2O7 is a well-known and actively used refractory system, which is a spinel material. We are used to the molecular formula of chromite (Fe,Mg)(Fe,Cr,Al)2O4, but its actual crystal chemical formula is another form, such as:

(Fe2+5.82Mg1.41Mn0.40Zn0.37)8(Cr12.96Fe3+2.71Al0.14Ti0.09)16O32 This is actually a high entropy material.

What are the current methods to make chromite (in its raw form - concentrate or ore, or in its processed form - molten or synthetic chromite/spinel) the most kinetically stable and resistant refractory material? At present, ceramic high-entropy materials can be synthesized by quite a number of methods, but the origins of all these methods fall into three categories:

- Mixing in solution (aqueous solution, organic solution);

- Mixing in dry matter (co-grinding, grinding);

- Mixing in melt (alloying of melt);

In choosing the best method, obviously, its economic rationality will be considered, but the cheapest method from a cost point of view is currently the wet chemical process in a closed water circulation system.

It is possible that we are wrong in some aspects and there are still some attractive methods waiting for us to develop on the traditional development path, which may increase the life of refractory linings by more than ten times in the next 30 years. But we also see that all refractory workers must master new water chemical processes, at least for the preparation of cutting-edge materials.