Abstract

Background: Refractory linings of cement rotary kilns are subjected to severe thermomechanical stresses of cranking, ovality, tyre hooping/migration, and uneven thermal distribution. An insistent demand is to discover the significance of spinel, hercynite, galaxite, and chromite in magnesia refractories, as well as their flexibilizing mechanisms.

Objectives: The objectives are to compare the fracture behavior of magnesia–spinel, – hercynite, –galaxite, and –chromite refractories and to unveil the flexibilizing mechanisms of different spinels.

Methods: The wedge splitting test is carried out to produce various fracture parameters. Their flexibilizing mechanisms are studied by performing microstructural observations and analyses.

Results: Various fracture parameters are obtained, including specific fracture energy, brittleness number, characteristic crack length, and thermal-shock resistance parameter. Generally, magnesia–hercynite bricks and magnesia–galaxite bricks have demonstrated the prominent advantages of fracture resistance, which are more flexible than magnesia–spinel bricks and magnesia–chromite bricks.

Conclusion: The flexibility of magnesia–spinel bricks is attributed to microcracks generated from different thermal behavior between spinel grains and surrounding periclase, which is recognized as the thermal-expansion mismatch mechanism. In magnesia–hercynite and magnesia–galaxite refractories, the active-ion-diffusion mechanism is predominant beyond similar microcracks, to drive the flexibility by the continuous diffusion of Fe²⁺, Mn²⁺, and Mg²⁺ during high-temperature burning and using processes. In magnesia–chromite bricks, the pore rims contribute to the flexibility as a silicate-migration mechanism, after silicate envelopes first arise around chromite grains and then vanish into the surrounding magnesia by the suction of capillary force during the burning process.

Keywords: Flexibilizing mechanism, magnesia, spinel, hercynite, galaxite, chromite.