Wasteless Synthesis and Properties of Highly Dispersed MgAl2O4 Based on Product of Thermal Activation of Gibbsite

The study showed that the interaction of the product of centrifugal thermal activation of gibbsite with an aqueous solution of magnesium nitrate at a cationic ratio Mg:Al = 1:2 leads to the formation of mixed double hydroxides both under hydrothermal treatment at 150 °C and at room temperature. The subsequent thermal treatment at 550 °C of the product of mild interaction leads to ~90% alumina-magnesia spinel and ~10% MgO, while the treatment of the hydrothermal interaction product leads to ~100% spinel with the stoichiometric composition MgAl2O4. The obtained spinel samples possess a high specific surface area (above 100 m2/g) and a hierarchical pore structure formed by the micron-level particles of different sizes (1–2 and 10–20 μm) consisting of ~70 nm crystallites with ~3 nm pores; the samples differ mostly in the total volume and quantitative ratio of the pores. The samples have Lewis acid sites of moderate strength on the surface, the amount of which is much lower to how it is when compared with a sample prepared by precipitation in that they also differ by quantity from each other as well (503 μmol/g for stoichiometric spinel and 304 μmol/g for sample with admixture of MgO). As the calcination temperature is raised to 850 °C, the acidity decreases—only weak Lewis acid sites are observed, the amount of which is also higher for stoichiometric spinel (161 and 39 μmol/g, respectively). The method proposed for the synthesis of alumina-magnesia systems provides a high dispersion and a much lower surface acidity for the oxides; in addition, it minimizes or completely excludes wash water, in distinction to the precipitation method.

In Situ Spinel Formation in a Smart Nano-Structured Matrix for No-Cement Refractory Castables

The hydration of an equimolar mixture of MgO and Al₂O₃ nano-powders has been proven to be an effective way to synthesize Mg₆Al₂CO₃(OH)₁₆∙4H₂O as a component of a nano-structured matrix and magnesia-alumina spinel precursor for high-performance cement-free corundum-spinel refractory castables. (Mg₃)–OH–brucite sites (417 °C) formed initially within the magnesia–alumina hydrating blended paste were replaced with (Mg₂Al)–OH and (Mg₃)–OH hydrotalcite sites, which were dehydroxylated at 420 °C and 322 °C, respectively. This reorganization was connected with the incorporation of anions and water molecules in the interlayer spacing of hydrotalcite, which was dehydrated at 234 °C. Hence, the thermal decomposition of a nano-structured matrix system containing mainly Mg₆Al₂CO₃(OH)₁₆∙4H₂O consists of a complex sequence of dehydration, dehydroxylation and decarbonization, and this finally leads to the formation of inverse spinel MgAl₂O₄ and periclase MgO through many intermediate stages containing the mixed tetrahedral-octahedral Al phase and MgO-like structure. Hence, the hydraulic bond that primarily existed was replaced by a ceramic bond at a relatively low temperature, i.e., 700 °C, where a spinel was formed. Important changes in oxygen coordination polyhedra around Al³⁺ in the dehydrated-dehydroxylated hydrotalcite occurred between 600 and 1100 °C.