Formation Of Sodium Aluminosilicate Research Articles

Processing of Low-Quality Gibbsite-Kaolinite Bauxites

The results of studies on the processing of gibbsite-kaolinite bauxite are presented. The developed technology includes preliminary chemical activation and thermal transformation during enrichment to obtain a concentrate suitable for processing by the Bayer method. As a result of the chemical activation of gibbsite-kaolinite bauxite in a solution of sodium bicarbonate, a change in the phase composition occurs, which made it possible to improve the results of gravity enrichment with the production of a coarse-grained gibbsite fraction. The transformation of bauxite in the temperature range of 900–1000 °C is explained by the decomposition reactions of siderite, gibbsite, kaolinite, calcite, dolomite and sodium ferro-sulfide oxide, as well as the formation of sodium aluminosilicate, hematite, quartz and the chemically stable phase of corundum. The optimum firing temperature of bauxite is 950 °C, after which, as a result of alkaline treatment during chemical enrichment, the extraction of SiO2 into solution was 74.9%. A silicon modulus of enriched bauxite 10.9 units was obtained. As a result of the autoclave leaching of gibbsite-kaolinite bauxite after a two-stage enrichment, the maximum extraction of alumina into solution was 87.4%. The yield of red mud during the processing of bauxite enriched and calcined at 950 °C was 37.62%. During the autoclave leaching of bauxite without enrichment, the yield of red mud was 71%.

Effect of PVA/SiO2 NPs Additive on the Structural, Durability, and Fire Resistance Properties of Geopolymers

This exertion introduces polyvinyl alcohol fiber/silica nanoparticles (poly vinyl alcohol (PVA)/SiO2 NPs) in the fly ash-based geopolymer at ambient curing temperature. The present study aims at investigating the structural properties (compressive, bond strength, fracture parameters (fracture toughness (KIc), crack mouth opening displacement (CMOD)), cyclic compression), durability (freeze-thaw), and fire resistivity of the newly developed PVA/SiO2 NPs mediated geopolymer. The outcomes suggest that geopolymers incorporated with 5% PVA fibers showed improved structural properties and durability as compared to other specimens. Investigation on the fire resistivity of the geopolymers exposed to different heating temperatures (400 °C, 600 °C, 800 °C), showed that geopolymers with PVA/SiO2 NPs significantly prevented the explosive concrete spalling. Microstructural studies confirmed that PVA fibers in the geopolymeric matrixes were well distributed and developed a fiber-bridging texture with improved performance. Addition of the nano-silica particles accelerated the heat evolution during the hydration process and the geopolymeric reaction (formation of sodium aluminosilicate N-A-S-H gel) at ambient curing environment.

Separation of aluminium and preparation of powdered DRI from lateritic iron ore based on direct reduction process

Lateritic iron ore has not been used effectively due to excess content of multiple metals. In this work, separation of aluminium from a high-aluminium lateritic iron ore was achieved by the process of ‘direct reduction with sodium sulfate-magnetic separation’, with a powdered direct reduced iron (DRI) produced. It is found that the presence of 12% sodium sulphate during reductive roasting significantly improves separation of iron and aluminium in magnetic separation: the total iron grade (TFe) of powdered DRI increases from 80.6 to 92.0% and the Al2O3 content decreases from 9.8 to 1.3% correspondingly. The presence of sodium sulphate results in formation of sodium aluminosilicates instead of FeAlO2. Moreover, sodium sulphate significantly promotes growth of metallic iron grains which is beneficial to sufficient liberation and separation of metallic iron grains from gangue minerals in grinding and magnetic separation.

Preparation of metakaolin based geopolymer coatings on metal substrates as thermal barriers

Metakaolin based geopolymer coatings on both stainless and mild steel substrates have been prepared. Adhesion of the geopolymer coatings strongly depended on composition. After curing at 70 °C geopolymer with Si:Al = 1 crystallized to zeolites LTA and A, while for compositions of Si:Al = 2 and 2.5 the coating was amorphous. For compositions with Si:Al = 2.5 the adhesive strength was > 3.5 MPa while for Si:Al = 1 and 2 only weak adhesion to the metal substrates was achieved. Heating these geopolymer compositions to 1000 °C resulted in formation of sodium aluminosilicate or nepheline. Thermal expansion measurements of the Si:Al = 2.5 composition revealed expansion of up to 6% at 800 °C, while for Si:Al = 1 and Si:Al = 2 compositions up to 4% shrinkage was observed.

Thermal activation and alkali dissolution of silicon from illite

TGA, DTA, FTIR, XRD and 29Si MAS NMR were used to investigate the thermal activation of illite. Illite experiences a series of thermal solid-state phase transformations. Dehydroxylation at 400∼700 °C forms dehydrated illite, which remains the layered framework of illite. From 700 °C to 1093 °C, Si–O tetrahedral framework remains unchanged. When heated above 1093 °C, the layered structure is destroyed and an amorphous silica-rich glass phase is formed. The mullitization occurs when the temperature exceeds 1100 °C. XRD and FTIR studies show that the silica in silica-rich glass phase dissolves in soda liquor, which indicates that silicon can be removed from illite after thermochemical activation (TCA) followed by alkali leaching. The formation of sodium aluminosilicates of Na 96Al 96Si 96O 384 and 0.95Na 2O · Al 2O 3 · 3.25SiO 2 · 4.79H 2O during alkali leaching may reduce the desilication to a great extent. Desilication tests after TCA show that the suitable activation conditions for illite are 1100∼1150°C and 90∼60 min. A desilication of 45% was obtained under the conditions for an ore sample bearing about 85% illite, 9% quartz and 5% muscovite.

Effects of In‐Tank Precipitation of Sodium Aluminosilicate on Uranium Chemistry

Experiments have been conducted to examine the fate of uranium during the formation of sodium aluminosilicate (NAS) when wastes containing high aluminate concentrations are mixed with wastes of high silicate concentration. Testing was conducted at varying degrees of uranium saturation. Testing examined typical tank conditions, e.g., stagnant, slightly elevated temperature (50°C). The results showed that under sub‐saturated conditions uranium is not removed from solution to any large extent in both simulant testing and actual tank waste testing. This aspect was not thoroughly understood prior to this work and was necessary to avoid criticality issues when actual tank wastes were aggregated. There are data supporting a small removal due to sorption of uranium on sites in the NAS. Above the solubility limit the data are clear that a reduction in uranium concentration occurs concomitant with the formation of aluminosilicate. This uranium precipitation is fairly rapid and ceases when uranium reaches its solubility limit. At the solubility limit, it appears that uranium is not affected, but further testing might be warranted.

Thermodynamic Modeling of Sodium Aluminosilicate Formation in Aqueous Alkaline Solutions

A thermodynamics-based model for sodium aluminosilicate formation in aqueous alkaline solutions was developed. Pitzer's method was adopted to calculate the activity of water and the activity coefficients of the other species in solution. The system under consideration contained the ions of Na+, Al(OH)4-, SiO32-, OH-, CO32-, SO42-, Cl-, and HS- dissolved in water and in equilibrium with two possible solid phases (sodalite dihydrate, Na8(AlSiO4)6Cl2·2H2O; hydroxysodalite dihydrate, Na8(AlSiO4)6(OH)2·2H2O) at 368.15 K. The equilibrium constants of sodalite dihydrate and hydroxysodalite dihydrate formation reactions were determined using the thermodynamic properties of the species involved. Property values that were not available in the literature were estimated by group contribution methods. The model calculates the molality of all species at equilibrium including the amount of solid precipitates. The results were found to be sensitive to the value of the equilibrium constant for hydroxysodalite dihydrate fo...

SYNTHESIS OF GRANULAR LOW-MODULUS ZEOLITES FROM METAKAOLIN USING MECHANOCHEMICAL ACTIVATION AND ULTRASONIC TREATMENT

The process of synthesis of granular low-modulus zeolites from a mixture of metakaolin and solid sodium hydroxide depending on the method of pre-treatment (mechanochemical activation or ultrasonic treatment) has been investigated. The vibration roller-ring mill VM-4 (vibration frequency 930 min–1) was used for mechanochemical activation (MCA) of the powder mixtures. Ultrasonic treatment of the aqueous suspensions was carried out in the ultrasonic disperser UD-20 (oscillation frequency 22 kHz). X-ray diffraction analysis, IR spectroscopy and scanning electron microscopy have been used for the study. It was established that Na2Al2O4 and SiO2 are formed in the system after MCA as a result of leaching, and after UST, Na2Al2O4 is formed only. It was shown that thermal treatment at 650 °C of the mixture after mechanochemical activation and ultrasonic treatment leads to the synthesis of sodium aluminosilicates of cubic syngony, but with different parameters of the crystal lattice. Thermal treatment of the mixture without treatment gives the formation of sodium aluminosilicates and silicon oxide. It was found that after hydrothermal crystallization in NaOH solution with a concentration of 2 mol l–1, the LTA zeolite is synthesized. Crystallization in an alkali solution with a concentration of 6 mol l–1 gives the formation of SOD. It was ascertained that the use of ultrasonic treatment of the initial mixture results in the synthesis of a maximum amount of LTA zeolite (80 %) and SOD (98 %).