E.F. Aglietti

Mechanochemical effects in kaolinite grinding. I. Textural and physicochemical aspects

Abstract This paper deals with textural and physicochemical alterations in kaolinite when subjected to a mechanochemical treatment by grinding in an oscillating mill. The techniques used: DTA, TGA, N2 adsorption, granulometric analysis, SEM, porosimetry, indicate the formation of agglomerates with high specific surface area and the gradual loss of crystallinity. It has been noted that the material maintains its chemical composition, thus evidencing the absence of thermal effects. The material obtained has a high density of surface residual charges and a high reactivity toward acid dissolution.

Zircon based ceramics by colloidal processing

Abstract Zircon is an excellent raw material to elaborate ceramics with good thermo-chemical properties. Also, mullite–zirconia materials can be prepared by reaction sintering of zircon–alumina mixtures. Concentrated aqueous alumina–zircon suspensions were prepared to study the effect of the slip properties on the density and microstructure of the compacts obtained by slip casting and pressure filtration at 8 MPa. The effect of the degree of dispersion and solid volume concentration on the viscosity of the suspension was studied. Optimization of the amount of polyacrylate dispersant added and pH of the suspension improved the solid loading and consequently the green and sintered properties of the cast samples.

Synthesis of Fe-FeAl2O4-Al2O3 by high-energy ball milling of Al-Fe3O4 mixtures

Physicochemical and structural changes induced by mechanical activation of Al–Fe3O4 mixtures are studied. After 37 min, the system undergoes a self-sustained reaction with formation of α-Fe, FeAl2O4 and α-Al2O3. The evolution of the composition follows a three-step reaction. The solid solution spinel Fe[Al2−xFex]O4, (0.13<x<0.29), is obtained through a mechanochemical-thermal route.

Mechanochemical effects in kaolinite grinding. II. Structural aspects

Abstract This study deals with the effect of mechanical treatment on the structure of a well-crystallized kaolinite by means of an oscillating mill. The evolution of crystallinity, crystal size and (001)spacing is evaluated with XRD techniques. An explanation of the evolution of the IR absorption bands and the relation thereof to what is observed by XRD is given. The structural disorder observed is compared to that of the naturally disordered kaolinites.

Effect of induced structural modifications on the physicochemical behavior of bentonite

Abstract The effect of impact and friction milling on the structure and texture of a bentonite, in its natural and in several monoionic forms (H + , Na + , Ca 2+ , Al 3+ ), is studied. An increase of treatment time shows progressive damage to the crystalline structure. The presence of different exchangeable ions does not exert a great influence on the behavior of the mineral. The physicochemical and structural alterations are studied by XRD, DTA, TGA, IR, CEC and swelling index.

MAGNESIA-AMMONIUM PHOSPHATE-BONDED CORDIERITE REFRACTORY CASTABLES: PHASE EVOLUTION ON HEATING AND MECHANICAL PROPERTIES

A cordierite refractory castable was developed using the MgO-NH4H2PO4 reaction. This castable was made with cordierite–mullite aggregates from scrap refractory material and a cement paste based on magnesia, calcined alumina, silica fume, and ammonium dihydrogen phosphate, which forms cordierite (2MgO·2Al2O3·5SiO2) during heating at high temperature. The mix with water was cast into steel molds; the cold setting occurs within 30 min. The set castables were thermally treated and the evolution of the phases was observed. Struvite (NH4·MgPO4·6H2O) was identified at room temperature; between 110 and 750 °C, the present phosphates were amorphous to X-ray diffraction (XRD). At 1100 °C, magnesium orthophosphate (Mg3(PO4)2) and aluminum orthophosphate (AlPO4) were present. At 1350 °C, the main crystalline phases were cordierite and mullite. Cold and hot flexural strength, thermal shock resistance, and physical properties were measured. The properties of magnesia–phosphate-bonded cordierite castables were compared with cordierite material obtained by conventional slip-casting method from aggregates, clay, talc, and calcined alumina.

Synthesis and characterization of aluminum carboxylate gels

Aluminum formate and basic acetate gels obtained from commercial pseudoboehmite were synthesized in this work. Phase evolution after successive thermal treatments was studied on the raw material and both gels. In both carboxylates the influence of aging time on the composition, the crystalline structure and the phase transition temperatures was studied by X-ray diffraction (XRD) and differential thermal analysis (DTA) and thermogravimetric analysis (TGA). Two different stages were observed in the thermal decomposition of both carboxylates: the first, corresponding to the decarboxilation and dehydration of the gels, was different for both cases because of the different type of carboxilic acids used; and the second, corresponding to the formation of transition aluminas, was the same whatever the precursor used. However, whenever the aging time was changed on any of the carboxylates, the mechanisms of the thermal transformations were affected.

Magnesia–phosphate bond for cold-setting cordierite-based refractories

A cold-setting refractory material was developed using the magnesia–phosphate reaction. A cement paste based on alumina, silica fume, magnesia and orthophosphoric acid or monoaluminum phosphate was designed to form cordierite–mullite during heating. This cement paste set at room temperature and MgHPO4·3H2O phase (newberyite) was observed, but amorphous phases were predominant. Two exothermic effects were detected during the setting process corresponding to the acid–base reaction of magnesia with phosphates and to the formation of bonding hydrates. At 1100 °C, C-AlPO4 was formed by reaction of alumina with orthophosphoric acid or monoaluminum phosphate. At 1350 °C, the principal crystalline phases were cordierite and mullite. A refractory concrete with the obtained cement paste and a cordierite–mullite aggregate (scrap refractory material) was prepared. At 1350 °C, this concrete had a thermal expansion coefficient of 1.0×10−6 °C−1 and a flexural strength of 10 MPa.

Magnetic and structural study of mechanochemical reactions in the Al-Fe3O4 system

The solid state reaction between Al and Fe3O4 (magnetite) using mechanochemical activation of powder mixtures under Ar atmosphere is studied. The phase evolution during the reaction is analyzed by X-ray diffraction (XRD), vibrating sample magnetometry (VSM), differential thermal analysis (DTA) and scanning electron microscopy (SEM). At 37 minutes of high-energy ball-milling the disappearance of reactive phases and the production of α-Fe, FeAl2O4 and α-Al2O3 is observed, together with significant changes in the magnetic behavior of the system. The composition and properties of samples heated up to 1200°C are also investigated. The behavior of the saturation magnetization Ms is interpreted on the basis of the formation of a variable composition spinel phase Fe [Alx Fe2−x] O4 with 0 ≤ x ≤ 2 and a canting effect due to the presence of Al3+ ions in the spinel structure.

Mechanochemical effects on the kinetics of zinc titanate formation

In this work, mixtures Zn-TiO2 (anatase) in molar ratio 1:1 were mechanochemically activated in air atmosphere, and submitted to thermal treatments in order to study its thermal transformations. The behavior of the system during the milling was followed by X-ray diffraction (XRD), differential thermal analyses (DTA) and thermogravimetric analyses (TGA).Mechanochemical activation produces a progressive loss in crystallinity of the starting powders, with simultaneous oxidation of metallic Zn. However, the formation of neither ZnTiO3 nor Zn2TiO4 could be detected. At temperatures above 600°C, the thermal treatments resulted in the formation of ZnTiO3 and Zn2TiO4, at lower temperatures and shorter holding times for samples activated during longer times. The non-activated mixture exhibited a very different behavior, yielding Zn2Ti3O8 and Zn2TiO4 without evidence of ZnTiO3 formation. The obtained results are explained on the basis of reaction mechanisms taking place in the activated and non-activated samples.