Silicate Mineral Eutectics with Special Reference to Lithium

In this paper, the system of natural mineral alkali fluxes used in typical mineral industry technologies was analyzed. The main objective was to reduce the melting temperature of the flux systems. Particular attention was paid to the properties of lithium aluminium silicates in terms of simplifying and accelerating the heat treatment process. In this area, an alkaline flux system involving lithium was analyzed. A basic flux system based on sodium potassium lithium aluminosilicates was analyzed; using naturally occurring raw materials such as spodumene, albite and orthoclase, an attempt was made to obtain the eutectic with the lowest melting point. Studies have shown that there are two eutectics in these systems, with about 30% spodumene content. The active influence of sodium feldspar was found.

Thermal Insulation of YSZ and Erbia-Doped Yttria-Stabilised Zirconia EB-PVD Thermal Barrier Coating Systems after CMAS Attack

The impact of small deposits of calcium–magnesium–aluminium silicates (CMAS) on the top of thermal barrier coatings (TBCs) made of yttria-stabilised zirconia (YSZ) produced via electron-beam physical vapour deposition (EB-PVD) is shown to play a role in the microstructural and chemical stability of the coatings; hence, it also affects the thermal insulation potential of TBCs. Therefore, the present work investigates the degradation potential of minor CMAS deposits (from 0.25 to 5 mg·cm−2) annealed at 1250 °C for 1 h on a novel Er2O3-Y2O3 co-stabilised ZrO2 (ErYSZ) EB-PVD TBC, which is compared to the standard YSZ coating. Due to the higher reactivity of ErYSZ coatings with CMAS, its penetration is limited in comparison with the standard YSZ coatings, hence resulting in a better thermal insulation of the former after ageing.

The influence of microbial mats on travertine precipitation in active hydrothermal systems (Central Italy)

ABSTRACTThe study of hydrothermal travertines contributes to the understanding of the interaction between physico-chemical processes and the role played by microbial mats and biofilms in influencing carbonate precipitation. Three active travertine sites were investigated in Central Italy to identify the types of carbonate precipitates and the associated microbial mats at varying physico-chemical parameters. Carbonate precipitated fabrics at the decimetre- to millimetre-scale and microbial mats vary with decreasing water temperature: a) at high temperature (55-44°C) calcite or aragonite crystals precipitate on microbial mats of sulphide oxidizing, sulphate reducing and anoxygenic phototrophic bacteria forming filamentous streamer fabrics, b) at intermediate temperature (44-40°C), rafts, coated gas bubbles and dendrites are associated with Spirulina cyanobacteria and other filamentous and rod-shaped cyanobacteria, c) low temperature (34-33°C) laminated crusts and oncoids in a terraced slope system are associated with diverse Oscillatoriales and Nostocales filamentous cyanobacteria, sparse Spirulina and diatoms. At the microscale, carbonate precipitates are similar in the three sites consisting of prismatic calcite (40-100 μm long, 20-40 μm wide) or acicular aragonite crystals organized in radial spherulites, overlying or embedded within biofilm EPS (Extracellular Polymeric Substances). Microsparite and sparite crystal size decreases with decreasing temperature and clotted peloidal micrite dominates at temperatures < 40°C, also encrusting filamentous microbes. Carbonates are associated with gypsum and Ca-phosphate crystals; EPS elemental composition is enriched in Si, Al, Mg, Ca, P, S and authigenic aluminium-silicates form aggregates on EPS.This study confirms that microbial communities in hydrothermal travertine settings vary as a function of temperature. Carbonate precipitate types at the microscale do not vary considerably, despite different microbial communities suggesting that travertine precipitation, driven by CO2 degassing, is influenced by biofilm EPS acting as template for crystal nucleation (EPS-mediated mineralization) and affecting the fabric types, independently from specific microbial metabolism.

Deposits in utilized Icelandic high-temperature systems

The most common scales in 7 utilized Icelandic high-temperature geothermal systems are calcite, amorphous silica, magnesium silicates, iron silicates, aluminium silicates and metal sulphides. They are mostly controlled by adjustment of temperature and pressure although chemical inhibition may be preferred due to production capacity. Reaming of wells by drilling and physical removal from surface equippment or acid cleaning may be needed.

Evaluation of Microwave Heating of Protective Coatings Used in the Lost Foam Technology

Presented are results of a preliminary research on determining a possibility to use microwave radiation for drying casting protective coatings applied on patterns used in the lost foam technology. Taken were measurements of permittivity ετ and loss factor tgδ at 2.45 GHz, as well as attempts were made of microwave drying of a protective coating based on aluminium silicates, applied on shapes of foamed polystyrene and rigid polymeric foam. Time and results of microwave drying were compared with the results obtained by drying at 50 °C by the traditional method commonly used for removing water from protective coatings. Analysis of the obtained drying kinetics curves demonstrated that selection of proper operation parameters of microwave equipment permits the drying time to be significantly shortened. Depending on kind of the pattern material, drying process of a protective coating runs in a different way, resulting in obtaining different quality of the dried coating.

Preparation and Rheological Characterization of Imogolite Hydrogels

Imogolite, one of the aluminium silicates, has a nanotube structure and has been known to form gel under alkaline condition. Imogolite nanotubes were synthesized in an acidic solution with various tube lengths by controlling the aging time from 1 d to 14 d. The length of the nanotubes grew from 100 nm to several μm as the aging time. Pure imogolite hydrogels were prepared by applying a salting-out method and centrifugation from its dispersed solutions with various tube lengths and solution pH. Imogolite hydrogel can be classified as the physically cross-linked one; the structure of the gel network is considered to be the entanglements and hydrogen bonding among nanotubes. The theoretical water contents of the prepared hydrogels were calculated as ∼99.7% in average. Gelation percentage significantly increased as the length of imogolite nanotubes. Whereas hydrogel prepared from 4 d aging sample showed the highest storage modulus of ∼970 Pa, it was found that the hydrogel could be prepared in the pH range from 6 to 10. The gel strength reached the highest value of 1000 Pa when the gel was prepared from the imogolite dispersed solution of pH 8. It could be explained by the surface charge variation of the imogolite.

Phase Composition and Morphology of Pottery Stone Microcrystalline Powders Synthesized by Hydrothermal Method

Pottery stone (PTS) microcrystalline powders were synthesized by a hydrothermal method using two concentrations of NaOH solution (2 M and 4 M) at 60, 80 and 120°C for 8 h in a Teflon- lined stainless steel autoclave. The phase composition and morphology of the samples were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The major phase compositions of the as-prepared PTS powders appeared in the XRD patterns are all silicate compounds such as tridymite, crystobalite, quartz, feldspar, albite, sodium silicate, analcime, hydroxyl-cancrinite and sodium aluminium silicates, depending on the NaOH concentration and the reaction temperature. Moreover, SEM micrographs showed the spherical polyhedral particles ranging from 8 to 14 µm in diameters and needle-like particles with a mean aspect ratio of 5.7 that obtained by the hydrothermal treatment operated at 120°C using 2 M and 4 M of NaOH, respectively.