The essential effect of iron ions in providing oxygen tothe human body by valency change in hemoglobin is wellknown. In a similar way, iron oxide is recognized asessential compound in cement and lime kiln refractoriesfor providing a satisfactory lifetime under varying kilnconditions. Investigations have shown that a definedamount of FeOx in basic bricks is rather advantageous,e. g. for thermoplastic stress relaxation or coating formation,without affecting the necessary requirementsregarding refractoriness, redox resistance, thermal shockresistance, etc. A measure for the thermoplastic behaviouris the deformation in the creep under compressiontest where the positive influence of iron oxide is shown.Early investigations describe the effect of chrome ore andhercynite, nowadays the Fe(II) and Fe(III) species containingpleonastic spinel show the most advantageousproperties. This is due to the fact that the iron ions arestable bound in the microstructure. This is shown by theredox test, where strength and volume change after alternationof reducing and oxidizing atmosphere is determined.Although a dependency is obvious, a high coldcrushing strength and modulus of rupture as well as thesimultaneous presence of both iron oxide species is advantageousconcerning thermoplastic stress relaxationand all other relevant properties. The graded presence ofiron species in a series of pleonaste-containing refractoriesallows the selection of adequate refractory materialregarding the requirement to cope with the influence ofhigher temperatures. It has to be concluded that a balancedpresence of both iron oxide species is quite beneficialfor the performance of rotary kiln bricks, in severalcases they outperform refractories based on very pure rawmaterials, especially in kilns with a high mechanical loadon the lining.
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