Abstract

The essential effect of iron ions in providing oxygen to the human body by valency change in hemoglobin is well known. In a similar way, iron oxide is recognized as essential compound in cement and lime kiln refractories for providing a satisfactory lifetime under varying kiln conditions. Investigations have shown that a defined amount of FeO(x) in basic bricks is rather advantageous, e. g. for thermoplastic stress relaxation or coating formation, without affecting the necessary requirements regarding refractoriness, redox resistance, thermal shock resistance, etc. A measure for the thermoplastic behavior is the deformation in the creep under compression test where the positive influence of iron oxide is shown. Early investigations describe the effect of chrome ore and hercynite, nowadays the Fe(II) and Fe(III) species containing pleonastic spinel show the most advantageous properties. This is due to the fact that the iron ions are stable bound in the microstructure. This is shown by the redox test, where strength and volume change after alternation of reducing and oxidizing atmosphere is determined. Although a dependency is obvious, a high cold crushing strength and modulus of rupture as well as the simultaneous presence of both iron oxide species is advantageous concerning thermoplastic stress relaxation as well as all other relevant properties. The graded presence of iron species in a series of pleonaste-containing refractories allows the selection of adequate refractory material regarding the requirement to cope with the influence of higher temperatures. It has to be concluded that a balanced presence of both iron oxide species is quite beneficial for the performance of rotary kiln bricks, in several cases they outperform refractories based on very pure raw materials, especially in kilns with a high mechanical load on the lining.