Refractory producers want to, need or must increase the use of secondary raw materials. They may be forced to do so by economic, ecologic or even legal causes. It is unclear, however, how impurities which are being carried over into refractory materials from secondary raw materials influence the behaviour of those refractory materials, either during the processing state or during their use in high temperature environments.
In order to overcome the lack of knowledge about the impact of minor concentrations of sodium, titanium, silicon, calcium and iron on the high temperature properties of refractories, said impurities were blend in primary high alumina raw materials (reactive alumina) in a well-defined way and used as a grain fraction in a refractory high alumina cement-free castable. High alumina cement-free refractory castables present the decisive advantage to be a relatively simple and convenient model with a well-defined and homogeneous composition, so that the effect of impurification (in the 0-45 μm fraction) should be easy to spot and investigate.
The selected impurities are typically process-related contaminations in tabular alumina primary raw materials or residue from previous industrial use in recycled raw materials. Based on the results from FactSage simulations, five, particularly critical, different combinations of different impurities were prepared. The total amount of added impurities in the “spiked” reactive alumina was set to 2 %. The preparation steps consist in 1) Mixing of reactive alumina with impurities, 2) Pelletizing for a proper connection, 3) Sintering for phase transformation/integration, 4) Screening/grinding to fraction of 0-45 μm. The quality of the precursors was then checked with X-ray fluorescence (XRF) for the concentration of impurities and homogeneity. Additionally, all precursors displayed similar particles size distribution, which is quite close to the particles size distribution of the fraction they substitute.
The thermomechanical behaviour of test pieces made of castables using “spiked” raw materials was finally characterized using Refractoriness under Load (RuL) and high temperature wedge splitting test measurements (WST). Thanks to WST, the stabile fracture process of test pieces is monitored and quantified, hence assessing the ability of the castable to resist damaging. The impurities free castable displayed a relative moderate strength and specific fracture energy that get weakened by increasing temperature. The addition of impurities to the castable almost systematically resulted in the deterioration of their mechanical properties and enhancement of their brittleness. One notable exception was the combination of iron and calcium that apparently even toughen the material and reduce its brittleness, which corelates with the fact that this combination of impurities is expected to form the lowest amount of liquid at the testing temperature and displayed the highest T0,5.
Results from the present investigations should help refractory producers to develop formulations suitable for specific applications with targeted high temperature properties and tailored admissible level of impurities, and hence promote the use of recycled raw materials.