The presented study is focused on optimization and characterization of a high-alumina refractory aggregate based on natural raw materials—kaolins, claystone, and mullite dust by-product (used to increase the alumina and mullite contents, respectively). In total, four individual formulas with the Al2O3 contents between 45 and 50 wt.% were designed; the samples were subsequently fired, both in a laboratory oven and an industrial tunnel furnace. The effects of repeated firing were examined during industrial pilot tests. Mineral and chemical compositions and microstructures, of both the raw materials and designed aggregates, were thoroughly investigated by the means of X-ray fluorescence spectroscopy, powder X-ray diffraction, and optical and scanning electron microscopies. Porosity, mineral composition, and mullite crystal-size development during the firing process were also studied. Based on the acquired results, the formula with the perspective to be used as a new mullite grog, featuring similar properties as the available commercial products, however, with reduced production expenses, was selected. The quality of grog determines to a large extent the properties of the final product. Hence, optimization of aggregates for specific refractories is of a great importance. The production of engineered aggregates provides the opportunity to utilize industrial by-products.

No abstract

Contemporary demand after high-quality ceramics leads to the depletion of raw material deposits. The industry also produces waste secondary materials that cannot be used directly for the desired purposes. It is therefore necessary to find other uses for these materials. It is common practice in the ceramics industry to stockpile fine fractions of refractory clays prior to their firing in a shaft kiln. These fractions have the size of 35 mm and less and when stored in a mound are facing a risk of weathering. Finding a purpose for these unused materials will markedly slow down fresh clay mining and make the production more eco-friendly and cost-effective. This paper seeks a suitable technology of treating fine fractions of BC (Brezina Clay), sometimes called FBC (Fine Brezina Clay), prior to its being fired into grog. The stockpiles hold tens of thousands of tons of these fractions. The properties of the fired clay are determined by the length of time for which the material has been stockpiled, moisture content, of the clay, as well as the briquetting pressure before firing in a shaft kiln. FBC contains approx. 41 % of aluminium oxide. This amount is not high enough for certain applications. The aluminium oxide content can be increased by the addition of bauxite, corundum, mullite, kaolin, clays rich in Al2O3, or technical Al2O3. The experiment described in this paper tested how the addition of technical Al2O3 affects the pore structure of fired FBC, which is later used as a grog in both shaped and non-shaped refractory products. The influence of firing temperature on the material's mineralogy was examined as well

Colloidal silica is used in many kinds of industry. It is an aqueous dispersion of SiO2 nanoparticles. SiO2 colloidal solutions are commercially available in different concentrations, with different particle sizes and are stabilized with different ions. Colloidal SiO2 was used in this study as a cement replacement in refractory castable. The present study, in its first stage, offers an assessment of five different SiO2 colloidal solutions. The particle size of the solutions was 15 nm, the particle concentration was 30% and 40% and the colloidal solutions were stabilized with Na+, OH− and Cl− ions. The effect of the colloidal solutions on selected characteristics of the refractory pastes and on their mineralogical composition after firing at 1000 °C and 1500 °C was described. The most suitable SiO2 colloidal solution from the first stage was subsequently used for the refractory castable test samples’ preparation in the second stage. Refractory castables, unlike paste, contain a coarse aggregate (grog) up to a grain size of 6 mm. Four types of coarse refractory grog were evaluated. Their effect on selected characteristics of the refractory castable and on its mineralogical composition after firing at 1000 °C and 1500 °C was described. The selected characteristics, within the scope of this study, include bulk density, apparent porosity, cold modulus of rupture and linear changes after firing. Finally, the study describes the effect of the sol particle concentration and the effect of pore size distribution on corrosion resistance and on the internal structure of the material. Mineral and chemical compositions and microstructures of both the raw materials and designed aggregates were thoroughly investigated by the means of X-ray fluorescence spectroscopy, powder X-ray diffraction and scanning electron microscopy. An analysis of the transition zone between corrosive media (K2CO3) and tested castables showed better corrosion resistance for a sol-gel castable than an ultra-low cement castable.