Increasing Cristobalitization and Mullitization in Fired Clays by Mechanochemical Activation and the Use of Al2O3- and SiO2-Containing Additives

Abstract

Results of the x-ray phase and derivatographic studies of refractory clays activated mechanochemically by means of planetary, vibratory, and screw grinding techniques are reported. The activation makes it possible to enhance cristobalitization and mullitization of the fired clays and to use lower firing temperatures. Good results giving the same effect is reached using Al 2 O 3 and SiO 2 -containings additives such as clays, oxidal, and nepheline syenite.The mixture for chamotte refractories is prepared using chamotte (fireclay), in a proportion of up to 60%, which is obtained by firing the raw clay. The raw clay includes aluminosilicate hydroxides: kaolinite Al 2 O 3 × 2SiO 2 × 2H 2 O (triclinic and monoclinic), halloyisite Al 2 O 3 × 2SiO 2 × nH 2 O (hexagonal and orthorhombic), and allophone mAl 2 O 3 × SiO 2 × nH 2 O (of as yet unknown crystal system). When calcined, they lose water and convert to mullite 3Al 2 O 3 × 2SiO 2 . Clay is fired into chamotte at 1400°C, this process is rather costly, and a technology that would permit one to decrease the firing temperature and to enhance cristobalitization and mullitization is a matter of serious concern. It goes without saying that the technology must be environment-friendly. All things considered, there is no point in introducing chemical agents into the technology or using very high pressures and temperatures. In materials subjected to mechanochemical activation, the conveyed energy is spent on "weakening" the crystal lattice which results in an increased specific surface. Here the only chemical means of initiating the process is atmospheric oxygen or water oxygen; viewed ecologically, this method of material treatment is preferable.Refractory clays are formed by redeposition of hydrated aluminosilicates in the crust of weathering; they most frequently occur in Carboniferous, Jurassic, Cretaceous and Cenozoic deposits. Clays, along with kaolinite, halloyisite, and allophane, may contain accessory species such as montmorillonite Al 2 [Si 4 O 10 ](OH) 2 × nH 2 O or mica, for example, illite (hydromica) K 0.7 Al 2 (Si, Al) 4 O 10 × (OH) 2 and other minerals. Halloyisite was found to be present in somewhat higher amounts in kaolinite clays of the Troshkovskoe deposit confined to the Jurassic [1].Subjected to mechanochemical activation, a-quartz was found to convert to b-quartz and to the high-temperature phase cristobalite [2]; that is, no sintering conditions were required for these modifications to co-exist with a-quartz [3]. As is known, cristobalite is a high-temperature polymorphic modification of quartz that forms from tridimite heated to above 1470°C and capable of persisting in a metastable state on cooling.In silica treated by mechanochemical in dry regime, defect structures in the form of Al 2 O 3 microcrystallites are formed on the grain surface [4]. This is seen in the contraction of coherent scattering regions, in the relative increase of microscopically distorted crystal lattice, and in the increased density of surface dislocations. The highly amorphiz...