Abstract

Magnesiumoxide is an excellent refractory raw material basis for the lining of the thermally highest loaded part of cement rotary kilns (CRK), the so called burning zone.  It is characterized by a high refractoriness and a high thermal expansion coefficient as well as thermal conductivity resulting in a rather poor thermal shock resistance and flexibility. To reduce the risk of brick hot face spalling, CRK-brick producers are using granular additives to prevent from crack formation. Refractory producers are talking of bricks with increased flexibility. Spinel-minerals are used  for refractory application such as Chromite, MA-spinel, hercynite or pleonaste. Consequently magnesia chromite bricks are continuously replaced, at first by MA-spinel bricks. In the 90’s the next step was the development of bricks based on hercynite, an FeO.Al2O3 spinel, as flexibiliser. Most recently cement rotary kiln bricks with addition of pleonaste appeared on the market, which mineralogically is a (MgO, FeO).Al2O3 spinel. It can be described as hercynite with a defined, additional MgO-content of 20-50%. In case of standard MA-spinel or pleonaste as additive mainly the thermal misfit (difference in linear expansion between the flexibiliser and the surrounding magnesia matrix) causes the formation of stress centers or even microcracks. If crack propagation in service occurs these stress centers or microcracks reduce the young’s modulus and increase the energy that is needed for crack propagation – the brick flexibility is increased. If hercynite is used the thermal misfit is enhanced by a diffusion effect of Fe into the surrounding matrix causing that hercynite is the most effective additive. In product development the V-moduls, the Young’s modulus and the specific fracture energy are used to characterize the brick’s flexibility.A test series with constant flexibiliser content proved that hercynite can be assumed as the most effective additive, while MA-Spinel and pleonaste are on a quite similar, but lower level. Accordingly, in order to reach a certain level of flexibility a higher amount of these additives need to be added to achieve a similar level of flexibilisation compared to hercynite. This negatively affects other important properties such as the resistance to clinker melt infiltration. Summarized it could be shown that hercynite is the most favourable additive to basic CRK-bricks leading to an optimum thermomechanical behavior combined with a maximized corrosion resistance for applications in the basic area of cement rotary kilns.